Display device and driving method thereof

ABSTRACT

The present invention provides a display device with reduced power consumption and that reduces changes in luminance, and perceptibility of flicker, and a driving method thereof. A display device according to an exemplary embodiment comprises: a display panel configured to display a still image and a motion picture; a signal controller configured to control signals for driving the display panel; and a graphics processing unit configured to transmit input image data to the signal controller, wherein the signal controller comprises a frame memory configured to store the input image data, and the display panel is driven at a first frequency when the motion picture is displayed and the display panel is driven at a second frequency that is lower than the first frequency when the still image is displayed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2011-0078796 filed in the Korean IntellectualProperty Office on Aug. 8, 2011, Korean Patent Application No.10-2011-0109915 filed in the Korean Intellectual Property Office on Oct.26, 2011, Korean Patent Application No. 10-2011-0114750 filed in theKorean Intellectual Property Office on Nov. 4, 2011, Korean PatentApplication No. 10-2011-0125169 filed in the Korean IntellectualProperty Office on Nov. 28, 2011, Korean Patent Application No.10-2012-0017618 filed in the Korean Intellectual Property Office on Feb.21, 2011, and Korean Patent Application No. 10-2012-0051496 filed in theKorean Intellectual Property Office on May 15, 2012, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a display device and a driving methodthereof. More particularly, the present invention relates to a displaydevice with reduced power consumption and that prevents deterioration ofvisibility, changes in luminance, and perceptibility of flicker, and adriving method thereof.

(b) Description of the Related Art

Currently, display devices are required for devices such as computermonitors, televisions, mobile phones, and the like, which are widelyused. The various types of display devices include cathode ray tubedisplay devices, liquid crystal displays, plasma display devices, andthe like.

A display device includes a graphics processing unit (GPU), a displaypanel, and a signal controller. The graphics processing unit transmitsimage data of a screen to be displayed on the display panel to thesignal controller. The signal controller then generates a control signalfor driving the display panel to transmit the control signal togetherwith the image data to the display panel, thereby driving the displaydevice.

Images displayed on the display panel are broadly classified into stillimages and motion pictures. The display panel is capable of displayingseveral frames per second, and if the image data included in the framesare identical, a still image is displayed. Conversely, if the image dataincluded in the frames are different, a motion picture is displayed.

In this case, because the signal controller receives the same image datafrom the graphics processing unit for every frame regardless of whetherthe display panel displays a motion picture or a still image, powerconsumption increases.

Recently, many methods for reducing power consumption of display deviceshave been researched. Among these, is a method in which a frame memoryis added to the signal controller, and the image data of the still imageis stored in a frame memory. The storage image data is provided to thedisplay panel while displaying the still image. The method is called apixel self-refresh (PSR) mode. Because the image data does not need tobe received from the graphics processing unit while displaying the stillimage, the graphics processing unit may be deactivated to reduce powerconsumption.

When being driven in the PSR mode, however, the addition of the framememory introduces a problem of increased power consumption.

The above information disclosed in this Background section is only forenhancement of understanding of the background and therefore it maycontain information that does not form the prior art that is alreadyknown to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

A display device with reduced power consumption and that preventsdeterioration of visibility, and a driving method thereof, are provided.

A display device that prevents a luminance change while reducing thepower consumption, and a driving method thereof are also provided.

Also, a display device that reduces perceptibility of flicker whilesimultaneously reducing the power consumption, and a driving methodthereof, are provided.

Also, a display device that prevents an increase of flicker due to anincrease of a leakage current while reducing the power consumption, anda driving method thereof, are provided.

A display device comprises: a display panel configured to display astill image and a motion picture; a signal controller configured tocontrol signals for driving the display panel; and a graphics processingunit configured to transmit input image data to the signal controller,wherein the signal controller comprises a frame memory configured tostore the input image data, and the display panel is driven at a firstfrequency when the motion picture is displayed and is driven at a secondfrequency that is lower than the first frequency when the still image isdisplayed.

The graphics processing unit may transmit a still image start signal anda still image end signal to the signal controller.

The signal controller may store the input image data in the framememory, may output storage image data stored in the frame memory to thedisplay panel at the second frequency, and may inactivate thetransmission of the input image data when the still image start signalis applied.

The signal controller may activate the transmission of the input imagedata and output the input image data to the display panel at the firstfrequency when the still image end signal is applied.

A length of a vertical blank period when the display panel is driven atthe second frequency may be longer than a length of the vertical blankperiod when the display panel is driven at the first frequency.

The display panel may be driven at a frequency that is higher than thesecond frequency and lower than the first frequency for an S1 frameafter the still image start signal is applied.

The length of the vertical blank period may be gradually increased forthe S1 frame.

The display panel may be driven at a frequency that is higher than thesecond frequency and lower than the first frequency for an S2 frameafter the still image end signal is applied.

The length of the vertical blank period may be gradually decreased forthe S2 frame.

The display panel may comprise: a substrate; a gate line and a data lineformed on the substrate; a switching element connected to the gate lineand the data line; and a pixel electrode connected to the switchingelement, and wherein the gate line may be applied with a gate signalcomprising a gate-on voltage and a gate-off voltage.

A clock frequency of the gate signal when the display panel is driven atthe second frequency may be lower than the clock frequency of the gatesignal when the display panel is driven at the first frequency.

A length of a vertical blank period when the display panel is driven atthe second frequency may be longer than a length of the vertical blankperiod when the display panel is driven at the first frequency.

The display panel may be driven with the second frequency until a frameapplied with the still image end signal is ended.

The display panel may comprise: a gate line and a data line; a gatedriver configured to drive a gate line; and a data driver configured todrive a data line, and the signal controller may transmit an STV signaland a CPV signal to the gate driver.

The signal controller may transmit the STV signal to the gate driver ata start position of one frame except for a position where the secondfrequency is changed into the first frequency.

The signal controller may be controlled for the widths of the CPV signalwhen the display panel is driven with the first frequency and the secondfrequency to be the same.

The signal controller may be controlled for the width of the CPV signalto have the same width as a clock signal of p times when the displaypanel is driven with a first frequency, and for the width of the CPVsignal to have the same width as a clock signal of q times less than ptimes when the display panel is driven with the second frequency.

The signal controller may gamma-correct the input image data when thedisplay panel is driven with the second frequency and may transmit thegamma-corrected image data to the display panel.

The signal controller may further comprise a frame counting unitconfigured to count the number of still image sequential frames inputtedbefore the still image end signal is applied after the still image startsignal is applied, and configured to count the number of motion picturesequential frames inputted until the still image start signal is appliedafter the still image end signal is applied.

The signal controller may store the input image data in the frame memoryand activate the transmission of the input image data when the number ofthe still image sequential frames is equal to or more than x. The signalcontroller may further activate the transmission of the input image datawhen the number of the motion picture sequential frames is equal to ormore than y.

The signal controller may output the storage image data stored in theframe memory to the display panel at the first frequency when the numberof the still image sequential frames is equal to or more than x, andoutput the input image data to the display panel at the second frequencywhen the number of the motion picture sequential frames is equal to ormore than y.

The display device may further comprise a light source unit configuredto irradiate light to the display panel, and a light source driverconfigured to control signals to drive the light source unit.

The light source driver may drive the light source unit at a first ratiowhen the display panel is driven at the first frequency, and drive thelight source unit at a second ratio when the display panel is driven atthe second frequency.

The second ratio may be lower than the first ratio when the displaypanel is a normally black mode, and the second ratio may be higher thanthe first ratio when the display panel is a normally white mode.

The signal controller may further comprise a signal receiving unitconfigured to transmit the input image data from the graphics processingunit, and a driving frequency selecting unit configured to select thefirst frequency when the still image is displayed and configured toselect the second frequency when the motion picture is displayed.

The light source driver may comprise a driving frequency receiving unitconfigured to receive a driving frequency of the display panel from thesignal controller, a light source unit driving ratio selecting unitconfigured to determine a driving ratio of the light source unitaccording to the driving frequency, and a light source driving signalgenerator configured to generate a signal for driving the light sourceaccording to the driving ratio of the light source unit.

The light source driver may constantly maintain a driving ratio of thelight source unit when the display panel is driven with the firstfrequency, and may periodically change the driving ratio of the lightsource unit when the display panel is driven with the second frequency.

The display panel may be a normally black mode, and the light sourcedriver may drive the light source unit with the first ratio when thedisplay panel is driven with the first frequency, and may drive thelight source unit with the first ratio and a ratio that is sequentiallydecreased from the first ratio when the display panel is driven with thesecond frequency.

The display panel may comprise a gate line and a data line, a gatedriver configured to drive a gate line, and a data driver configured todrive a data line, and the signal controller may transmit an STV signalto the gate driver at a start position of every frame.

The light source driver may drive the light source unit with the firstratio as a position where the STV signal is transmitted when the displaypanel is driven with the second frequency, and may drive the lightsource unit with a ratio that is sequentially decreased from the firstratio before a next STV signal is transmitted.

A transmission cycle of the STV signal when the display panel is drivenwith the first frequency may be the same as a change cycle of a drivingratio of the light source unit when the display panel is driven with thesecond frequency.

The display panel may be a normally white mode, and the light sourcedriver may drive the light source unit with the first ratio when thedisplay panel is driven with the first frequency, and may drive thelight source unit with the first ratio and a ratio that is sequentiallyincreased from the first ratio when the display panel is driven with thesecond frequency.

The display panel may comprise: a substrate; a gate line, a data line,and a storage electrode line formed on the substrate; a first switchingelement connected to the gate line and the data line; and a storagecapacitor connected to the first switching element and the storageelectrode line, wherein when the display panel i driven with the firstfrequency, a common voltage input to the storage electrode line has aconstant value, and when the display panel is driven with the secondfrequency, the common voltage may have a value that is changed accordingto time.

The display panel may further comprise: a second switching element and athird switching element formed between the storage electrode line andthe storage capacitor; and a storage electrode control line formed onthe substrate, wherein each of the second switching element and thethird switching element comprises a control terminal, an input terminal,and an output terminal, the input terminals of the second switchingelement and the third switching element are connected to the storageelectrode line, the output terminals of the second switching element andthe third switching element are connected to the storage capacitor, thecontrol terminal of the second switching element is connected to thegate line, and the control terminal of the third switching element isconnected to the storage electrode control line.

When the display panel is driven at the second frequency, the commonvoltage may have a first voltage in at first time and have a secondvoltage that is higher than the first voltage at a second time.

One frame may comprise an effective period in which the image data istransmitted and a vertical blank period in which the image data is nottransmitted, the first period may be the effective period, and thesecond period may be the vertical blank period.

The control voltage input to the storage electrode control line may havea gate-off voltage in the first period and a gate-on voltage in thesecond period.

When the display panel is driven with the second frequency, the commonvoltage may have a third voltage that is higher than the second voltagein the third period.

One frame may comprise an effective period in which the image data istransmitted and a vertical blank period in which the image data is nottransmitted, the first period may be the effective period, the secondperiod may be a portion of the vertical blank period, and the thirdperiod may be a remaining portion of the vertical blank period.

When the display panel is driven with the second frequency, the commonvoltage may have the first voltage in the first period and may swingwith the first voltage and the second voltage that is higher than thefirst voltage in the second period.

The common voltage may be gradually changed with a value between thefirst voltage and the second voltage when the first voltage is changedinto the second voltage.

The display device may further comprise a gate driver configured todrive the gate line and a data driver configured to drive the data line,and the signal controller may comprise a calculator configured tocalculate a representative value of storage image data stored in theframe memory, a line memory configured to store the representativevalue, and a kick-back corrector configured to generate auxiliary imagedata by correcting the representative value according to a kick-backvoltage. The data driver may apply an auxiliary voltage corresponding tothe auxiliary image data to the data lines in a vertical blank range atthe time of displaying the still image.

A plurality of data lines may be provided, and the calculator maycalculate a representative value of the storage image data for each dataline.

The representative value may be an average gray value of the storageimage data.

The representative value may be an average gray value of upper t bits ofthe storage image data.

The representative value may be a middle value of a maximum gray valueand a minimum gray value of the storage image data.

The auxiliary image data may be generated by Ga=Gr−dG (Ga: a gray valueof the auxiliary image data, Gr: the representative value, dG: akick-back correction gray value depending on the representative value).

The kick-back correction gray value may be a value stored in a look-uptable or calculated by a function.

When the kick-back correction gray value is a value calculated by afunction, the function may be generated by linear interpolation by usinga kick-back correction gray value at a minimum gray, a kick-backcorrection gray value at a maximum gray, and a gray value when themagnitude of the kick-back correction gray value is a maximum.

The display panel may comprise: gate lines and data lines; a switchingelement of which a control terminal is connected to the gate line and aninput terminal is connected to the data line; and a pixel electrodeconnected to an output terminal of the switching element, wherein a gatesignal comprising gate-on voltage and gate-off voltage may be applied tothe gate line, and the gate-off voltage when the display panel is drivenat the second frequency may have the following range:Va−0.2|Va|≦Voff2≦Va+0.2|Va| (Voff2: the gate-off voltage when thedisplay panel is driven at the second frequency, Va: voltage of thecontrol terminal of the switching element when leakage current flowingbetween the input terminal and the output terminal of the switchingelement when the positive pixel voltage is applied to the pixelelectrode and leakage current flowing between the input terminal and theoutput terminal of the switching element when the negative pixel voltageis applied to the pixel electrode are the same).

The gate-off voltage when the display panel is driven at the secondfrequency may have the following range: Va−0.1|Va|≦Voff2≦Va+0.1|Va|(Voff2: the gate-off voltage when the display panel is driven at thesecond frequency, Va: voltage of the control terminal of the switchingelement when leakage current flowing between the input terminal and theoutput terminal of the switching element when the positive pixel voltageis supplied to the pixel electrode and leakage current flowing betweenthe input terminal and the output terminal of the switching element whenthe negative pixel voltage is supplied to the pixel electrode are thesame).

The gate-off voltage when the display panel is driven at the firstfrequency may have the following range: Va−0.2|Va|≦Voff1≦Va+0.2|Va|(Voff1: the gate-off voltage when the display panel is driven at thefirst frequency, Va: voltage of the control terminal of the switchingelement when leakage current flowing between the input terminal and theoutput terminal of the switching element when the positive pixel voltageis applied to the pixel electrode and leakage current flowing betweenthe input terminal and the output terminal of the switching element whenthe negative pixel voltage is applied to the pixel electrode are thesame).

The gate-off voltage when the display panel is driven at the firstfrequency may have the following range: Va−0.1|Va|≦Voff1≦Va+0.1|Va|(Voff1: the gate-off voltage when the display panel is driven at thefirst frequency, Va: voltage of the control terminal of the switchingelement when leakage current flowing between the input terminal and theoutput terminal of the switching element when the positive pixel voltageis applied to the pixel electrode and leakage current flowing betweenthe input terminal and the output terminal of the switching element whenthe negative pixel voltage is applied to the pixel electrode are thesame).

The gate-off voltage when the display panel is driven at the firstfrequency may be the same as the gate-off voltage when the display panelis driven at the second frequency.

The gate-off voltage when the display panel is driven at the firstfrequency may be lower than the gate-off voltage when the display panelis driven at the second frequency.

The display device may further comprise a gate driver configured todrive the gate line and a data driver configured to drive the data line,wherein the signal controller may store the input image data in theframe memory, apply storage image data stored in the frame memory to thedata driver, and inactivate the transmission of the input image data,when the still image start signal is applied.

The transmission of the input image data may be activated and the inputimage data may be applied to the data driver when the still image endsignal is applied.

The gate driver may be attached at one side of the display panel.

The gate driver may be mounted in the display panel together with thegate line, the data line, and the switching element.

In another aspect, a method for driving a display device comprising adisplay panel displaying a moving picture and a still image, and asignal controller controlling signals to drive the display panelcomprises: transmitting input image data and driving a display panelwith a first frequency; applying a still image start signal; changing adriving frequency of the display panel into a second frequency that islower than the first frequency; applying a still image end signal; andchanging a driving frequency of the display panel into the firstfrequency.

The input image data may be stored in the frame memory, the transmissionof the input image data may be inactivated, and storage image datastored in the frame memory may be outputted to the display panel at thesecond frequency, when the still image start signal is applied.

The transmission of the input image data may be activated and the inputimage data may be outputted to the display panel at the first frequency,when the still image end signal is applied.

A length of a vertical blank period when the display panel is driven atthe second frequency may be longer than a length of the vertical blankperiod when the display panel is driven at the first frequency.

The display panel may be driven with a frequency that is higher than thesecond frequency and is lower than the first frequency during an S1frame after the still image start signal is applied.

A length of the vertical blank period may be gradually increased duringthe S1 frame.

The display panel may be driven with a frequency that is higher than thesecond frequency and is lower than the first frequency during an S2frame after the still image end signal is applied.

A length of the vertical blank period may be gradually decreased duringthe S2 frame.

The display panel may comprise: a substrate; a gate line and a data lineformed on the substrate; a switching element connected to the gate lineand the data line; and a pixel electrode connected to the switchingelement, wherein the gate line is applied with a gate signal comprisinga gate-on voltage and a gate-off voltage.

A clock frequency of the gate signal when the display panel is driven atthe second frequency may be lower than the clock frequency of the gatesignal when the display panel is driven at the first frequency.

A length of a vertical blank period when the display panel is driven atthe second frequency may be longer than a length of the vertical blankperiod when the display panel is driven at the first frequency.

The display panel may be driven with the second frequency until theframe applied with the still image end signal ends, and the drivingfrequency of the display panel may be changed into the first frequencyin a frame after the still image end signal is applied.

The display panel may comprise a gate line and a data line, a gatedriver configured to drive the gate line, and a data driver configuredto drive the data line, wherein the signal controller may transmit anSTV signal and a CPV signal to the gate driver.

The signal controller may transmit the STV signal to the gate driver atthe start position of every frame except for a position where the secondfrequency is changed into the first frequency.

The signal controller may be controlled for the width of the CPV signalto be the same when the display panel is driven with the first frequencyand the second frequency.

The signal controller may be controlled for the width of the CPV signalto have the same width as the clock signal of p times when the displaypanel is driven with the first frequency, and for the width of the CPVsignal to have the same width as the clock signal of q times less than ptimes when the display panel is driven with the second frequency.

The signal controller may gamma-correct image data when the displaypanel is driven with the second frequency, and may transmit thegamma-corrected image data to the display panel.

The method may further comprise: counting the number of still imagesequential frames inputted before the still image end signal is appliedafter the still image start signal is applied; and counting the numberof motion picture sequential frames inputted until the still image startsignal is applied after the still image end signal is applied.

The signal controller may store the input image data in the frame memoryand may inactivate the transmission of the input image data when thenumber of the still image sequential frames is equal to or more than x,and may activate the transmission of the input image data when thenumber of the motion picture sequential frames is equal to or more thany.

The signal controller may output the storage image data stored in theframe memory to the display panel at the second frequency when thenumber of the still image sequential frames is equal to or more than x,and may output the input image data to the display panel at the firstfrequency when the number of the motion picture sequential frames isequal to or more than y.

When the display panel is driven at the first frequency, the lightsource unit may be driven at the first ratio, and when the display panelis driven at the second frequency, the light source unit may be drivenat the second ratio.

When the display panel is a normally black mode, the second ratio mayhave a lower value than the first ratio, and when the display panel is anormally white mode, the second ratio may have a higher value than thefirst ratio.

The driving ratio of the light source unit according to the drivingfrequency of the display panel may be selected by using a look-up tableor a function.

The conversion of the driving frequency of the display panel and thedriving ratio of the light source unit may be performed in a verticalblank period.

When the display panel is driven with the first frequency, a drivingratio of the light source unit may be constantly maintained, and whenthe display panel is driven with the second frequency, the driving ratioof the light source unit may be periodically changed.

The display panel may be a normally black mode, when the display panelis driven with the first frequency, the light source unit may be drivenwith the first ratio, and when the display panel is driven with thesecond frequency, the light source unit may be driven with the firstratio and a ratio that is sequentially decreased from the first ratio.

The display panel may comprise a gate line and a data line, the displaydevice may further comprise a gate driver configured to drive the gateline and a data driver configured to drive the data line, and the signalcontroller may transmit an STV signal to the gate driver at the startposition every frame.

When the display panel is driven with the second frequency, the lightsource unit may be driven with the first ratio at a positiontransmitting the STV signal, and the light source unit may be drivenwith a ratio that is sequentially decreased from the first ratio beforetransmission of a next STV signal.

A transmission cycle of the STV signal when the display panel is drivenwith the first frequency may be the same as a change cycle of thedriving ratio of the light source unit when the display panel is drivenwith the second frequency.

The display panel may be a normally white mode, when the display panelis driven with the first frequency, the light source unit may be drivenwith the first ratio, and when the display panel is driven with thesecond frequency, the light source unit may be driven with the firstratio and a ratio that is sequentially increased from the first ratio.

When the display panel is driven with the first frequency, the signalcontroller may apply a common voltage having a constant value to thedisplay panel, and when the display panel is driven with the secondfrequency, the signal controller may apply a common voltage having avalue that is changed to the display panel.

When the display panel is driven with the second frequency, the signalcontroller may apply a common voltage having a first voltage in thefirst period and a second voltage that is higher than the first voltagein the second period.

One frame may comprise an effective period in which the image data istransmitted and a vertical blank period in which the image data is nottransmitted, the first period may be the effective period, and thesecond period may be the vertical blank period.

When the display panel is driven with the second frequency, the signalcontroller may apply a common voltage having a third voltage that ishigher than the second voltage in the third period to the display panel.

One frame may comprise an effective period in which the image data istransmitted and a vertical blank period in which the image data is nottransmitted, the first period may be the effective period, the secondperiod may be a portion of the vertical blank period, and the thirdperiod may be a remaining portion of the vertical blank period.

When the display panel is driven with the second frequency, the signalcontroller may apply a common voltage having the first voltage in thefirst period and swinging between the first voltage and the secondvoltage that is higher than the first voltage in the second period.

The common voltage may be gradually changed with a value between thefirst voltage and the second voltage when the first voltage is changedinto the second voltage.

The method may further comprise: storing the input image data to theframe memory if the still image start signal is applied; calculating arepresentative value of the storage image data stored to the framememory; correcting the representative value according to the kick-backvoltage to generate and auxiliary image data; and applying the auxiliaryvoltage corresponding to the auxiliary image data to the data line inthe vertical blank period.

The data line may be provided in plural, and the representative value ofthe storage image data may be calculated for each data line.

The representative value may be an average gray value of the storageimage data.

The representative value may be an average gray value of upper t bits ofthe storage image data.

The representative value may be a middle value between a maximum grayvalue and a minimum gray value of the storage image data.

The auxiliary image data may be generated by Ga=Gr−dG (Ga: a gray valueof the auxiliary image data, Gr: the representative value, dG: akick-back correction gray value depending on the representative value).

The kick-back correction gray value may be a value stored in a look-uptable or calculated by a function.

When the kick-back correction gray value is a value calculated by thefunction, the function may be generated by linear interpolation by usinga kick-back correction gray value at a minimum gray, a kick-backcorrection gray value at a maximum gray, and a gray value when themagnitude of the kick-back correction gray value is a maximum.

When the display panel is driven at the second frequency, the signalcontroller may apply a gate-off voltage having a rangeVa−0.2|Va|≦Voff2≦Va+0.2|Va| (Voff2: the gate-off voltage when thedisplay panel is driven at the second frequency, Va: voltage of thecontrol terminal of the switching element when leakage current flowingbetween the input terminal and the output terminal of the switchingelement when the positive pixel voltage is applied to the pixelelectrode and leakage current flowing between the input terminal and theoutput terminal of the switching element when the negative pixel voltageis applied to the pixel electrode are the same) to the display panel.

When the display panel is driven at the second frequency, the signalcontroller may apply a gate-off voltage having a rangeVa−0.1|Va|≦Voff2≦Va+0.1|Va| (Voff2: the gate-off voltage when thedisplay panel is driven at the second frequency, Va: voltage of thecontrol terminal of the switching element when leakage current flowingbetween the input terminal and the output terminal of the switchingelement when the positive pixel voltage is applied to the pixelelectrode and leakage current flowing between the input terminal and theoutput terminal of the switching element when the negative pixel voltageis applied to the pixel electrode are the same) to the display panel.

When the display panel is driven at the first frequency, the signalcontroller may apply a gate-off voltage having a rangeVa−0.2|Va|≦Voff1≦Va+0.2|Va| (Voff1: the gate-off voltage when thedisplay panel is driven at the first frequency, Va: voltage of thecontrol terminal of the switching element when leakage current flowingbetween the input terminal and the output terminal of the switchingelement when the positive pixel voltage is applied to the pixelelectrode and leakage current flowing between the input terminal and theoutput terminal of the switching element when the negative pixel voltageis applied to the pixel electrode are the same).

When the display panel is driven at the first frequency, the signalcontroller applies a gate-off voltage having a rangeVa−0.1|Va|≦Voff1≦Va+0.1|Va| (Voff1: the gate-off voltage when thedisplay panel is driven at the first frequency, Va: voltage of thecontrol terminal of the switching element when leakage current flowingbetween the input terminal and the output terminal of the switchingelement when the positive pixel voltage is applied to the pixelelectrode and leakage current flowing between the input terminal and theoutput terminal of the switching element when the negative pixel voltageis applied to the pixel electrode are the same).

The gate-off voltage when the display panel is driven at the firstfrequency may be the same as the gate-off voltage when the display panelis driven at the second frequency.

The gate-off voltage when the display panel is driven at the firstfrequency may be lower than the gate-off voltage when the display panelis driven at the second frequency.

In the display device and the driving method thereof according to anexemplary embodiment, the still image is displayed with a lowerfrequency compared with the motion picture, thereby reducing powerconsumption. The frequency of the still image is set lower than apredetermined value such that power consumption shows a net reductionafter considering power requirements of the frame memory.

Also, the still image is further displayed at a position where the lowerstill image frequency is converted into the faster motion picturefrequency such that no switching defect is noticeable when the frequencychange occurs.

Also, although the frequency is changed, the width of the CPV signal isset to be constant or is gamma-corrected such that any visibility defectdue to the frequency change may be prevented.

Also, the dimming driving for the light source unit is performed duringthe frequency change such that any luminance change associated with thefrequency change may be prevented.

Also, when the number of the still image sequential frames is more thana predetermined number or the number of the motion picture sequentialframes is more than another predetermined number, the driving frequencyof the display panel is changed such that any luminance changeassociated with the frequency change may be prevented.

Also, when driving the display panel with the second frequency, thecommon voltage is changed to change the luminance, and thereby flickeris imperceptible.

Also, when driving the display panel with the second frequency, thevalue representing the storage image data for each data line iscalculated in the vertical blank period and the auxiliary voltagecorresponding to the kick-back correction value is applied to the dataline such that the leakage current may be reduced, thereby reducingflicker.

Further, by setting a range of gate-off voltage based on the time whenleakage current of the switching element when the positive pixel voltageis applied to the pixel electrode is the same as leakage current of theswitching element when the negative pixel voltage is applied to thepixel electrode, it is possible to reduce flicker.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a display device according to a firstexemplary embodiment.

FIG. 2 is a block diagram illustrating a signal controller of thedisplay device according to the first exemplary embodiment.

FIG. 3 is a view of control signals of a display device according to thefirst exemplary embodiment.

FIG. 4 is an alternative view of control signals of a display deviceaccording to the first exemplary embodiment.

FIG. 5 is a diagram illustrating a DE signal and a Vsync signal used ina display device according to the first exemplary embodiment.

FIG. 6 is a diagram illustrating a gate signal and an STV signal when adisplay panel is driven at a first frequency in the display deviceaccording to the first exemplary embodiment.

FIG. 7 to FIG. 9 are diagrams illustrating a gate signal and an STVsignal when a display panel is driven at a second frequency in thedisplay device according to the first exemplary embodiment.

FIG. 10 is a diagram illustrating a clock signal and a CPV signal when adisplay panel is driven at a first frequency in the display deviceaccording to the first exemplary embodiment.

FIG. 11 is a diagram illustrating a clock signal and a CPV signal when adisplay panel is driven at a second frequency in the display deviceaccording to the first exemplary embodiment.

FIG. 12 is a flowchart showing a method of amending image data in adisplay device according to the first exemplary embodiment.

FIG. 13 is a block diagram of a display device according to the secondexemplary embodiment.

FIG. 14 is a block diagram illustrating a light source driver of thedisplay device according to the second exemplary embodiment.

FIG. 15 is a flowchart illustrating a driving method of the displaydevice according to the second exemplary embodiment.

FIG. 16 to FIG. 18 are block diagrams of the signal controllerillustrating the driving method of the display device according to thesecond exemplary embodiment for each step in sequence.

FIG. 19 and FIG. 20 are block diagrams of the light source driverillustrating the driving method of the display device according to thefirst exemplary embodiment for each step in sequence.

FIG. 21 is a block diagram illustrating a signal controller according toa third exemplary embodiment.

FIG. 22 is a flowchart illustrating a driving method of the displaydevice according to the third exemplary embodiment.

FIG. 23 to FIG. 26 are block diagrams of the signal controllerillustrating the driving method of the display device according to thethird exemplary embodiment for each step in sequence.

FIG. 27 is a flowchart of a driving method of a display device accordingto the fourth exemplary embodiment.

FIG. 28 and FIG. 29 are views of an STV signal and a light source unitdriving ratio of a display device according to the fourth exemplaryembodiment.

FIG. 30 is an equivalent circuit diagram for one pixel of a displaydevice according to the fifth exemplary embodiment.

FIG. 31 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe fifth exemplary embodiment.

FIG. 32 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe fifth exemplary embodiment.

FIG. 33 is an equivalent circuit diagram for one pixel of a displaydevice according to the sixth exemplary embodiment.

FIG. 34 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe sixth exemplary embodiment.

FIG. 35 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe sixth exemplary embodiment.

FIG. 36 is a graph illustrating power consumption according to drivingfrequency.

FIG. 37 is a graph illustrating voltage of one terminal of a storagecapacitor when a display panel is driven at 60 Hz.

FIG. 38 is a graph illustrating voltage of one terminal of a storagecapacitor when a known display panel is driven at 10 Hz.

FIG. 39 is a graph illustrating voltage of one terminal of a storagecapacitor when a display panel according to the fifth exemplaryembodiment is driven at 10 Hz.

FIG. 40 is a block diagram of a signal controller of the display deviceaccording to the seventh exemplary embodiment.

FIG. 41 is a graph illustrating kick-back voltage depending on a grayvalue of image data.

FIG. 42 is a graph illustrating a kick-back correction gray valuedepending on the gray value of the image data.

FIG. 43 is an equivalent circuit diagram for one pixel of the displaydevice according to the seventh exemplary embodiment.

FIG. 44 is a diagram illustrating leakage current when a predeterminedvoltage is applied during a vertical blank range in the display deviceaccording to the exemplary embodiment.

FIG. 45 is a view of one pixel of a display device according to theeighth exemplary embodiment.

FIG. 46 is a graph illustrating current between an input terminal and anoutput terminal according to gate voltage in a switching element of thedisplay device according to the eighth exemplary embodiment.

FIG. 47 is a diagram illustrating a luminance characteristic when astill image is displayed in a display device according to the relatedart.

FIG. 48 is a diagram illustrating a luminance characteristic when astill image is displayed in the display device according to the eighthexemplary embodiment.

FIG. 49 is a graph illustrating a flicker value according to a gate-offvoltage value when a still image is displayed in the display deviceaccording to the eighth exemplary embodiment.

FIG. 50 is a graph illustrating intensity of light emitted from adisplay panel over time.

FIG. 51 is a diagram illustrating equipment used for flickermeasurement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure will be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsare shown. As those skilled in the art would realize, the describedembodiments may be modified in various different ways, all withoutdeparting from the spirit or scope of the present invention.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like reference numerals designate likeelements throughout the specification. It will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element, orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present.

First, a display device according to the first exemplary embodiment willbe described below with reference to the accompanying drawings.

FIG. 1 is a block diagram of a display device according to the firstexemplary embodiment.

As shown in FIG. 1, the display device according to the first exemplaryembodiment includes a display panel 300 displaying an image, a signalcontroller 600 controlling signals for driving the display panel 300,and a graphics processing unit 700 transmitting input image data to thesignal controller 600.

The display panel 300 receives image data DAT from the signal controller600 to display a still image and a motion picture. If a plurality ofsequential frames have the same image data DAT, the still image isdisplayed, and if the plurality of sequential frames have differentimage data DAT, the motion picture is displayed.

The display panel 300 includes a plurality of gate lines G1-Gn and aplurality of data lines D1-Dm, the plurality of gate lines G1-Gn extendin a horizontal direction, and the plurality of data lines D1-Dm extendin a vertical direction while crossing the plurality of gate linesG1-Gn.

One of the gate lines G1-Gn and one of the data lines D1-Dm areconnected with one pixel, and a switching element Q connected with oneof the gate lines G1-Gn and one of the data lines D1-Dm is included ineach pixel. A control terminal of the switching element Q is connectedto the gate lines G1-Gn, an input terminal thereof is connected with thedata lines D1-Dm, and an output terminal is connected with a liquidcrystal capacitor CLC and a storage capacitor CST.

The display panel 300 of FIG. 1 is shown as a liquid crystal panel,however the display panel 300 may be one of the various types of displaypanels, such as, for example, an organic light emitting panel, anelectrophoretic display panel, or a plasma display panel, as well as theliquid crystal panel.

The signal controller 600 processes the input image data and the controlsignals so as to be suitable for the operation condition of the liquidcrystal panel 300 in response to the input image data received from thegraphics processing unit 700 and the control signals thereof, forexample, a vertical synchronization signal Vsync, a horizontalsynchronization signal Hsync, a main clock signal MCLK, a data enablesignal DE, and the like, and then generates and outputs a gate controlsignal CONT1 and a data control signal CONT2.

The gate control signal CONT1 includes a vertical synchronization startsignal STV instructing an output start of a gate-on pulse (high time ofa gate signal GS), a gate clock signal CPV controlling an output time ofthe gate-on pulse, and the like.

The data control signal CONT2 includes a horizontal synchronizationstart signal STH instructing an input start of the image data DAT, aload signal TP instructing application of the corresponding data voltageto the data lines D1-Dm, and the like.

The graphics processing unit 700 transmits the input image data to thesignal controller 600. When the display panel 300 displays the motionpicture, the graphics processing unit 700 transmits the input image datato the signal controller 600 every frame. When the display panel 300displays the still image, because the signal controller 600 stores theinput image data received from the graphics processing unit 700 totransmit the input image data to the display panel 300, the graphicsprocessing unit 700 does not transmit the input image data to the signalcontroller 600. That is, when the display panel 300 displays the stillimage, the graphics processing unit 700 is inactivated.

The graphics processing unit 700 transmits a still image start signal tothe signal controller 600 at the conversion time when the input imagedata displaying the motion picture is transmitted, and then the inputimage data displaying the still image is transmitted. Further, thegraphics processing unit 700 transmits a still image end signal to thesignal controller 600 at the conversion time when the input image datadisplaying the still image is transmitted, and then the input image datadisplaying the motion picture is transmitted.

The display device according to the first exemplary embodiment mayfurther include a gate driver 400 driving the gate lines G1-Gn and adata driver 500 driving the data line D1-Dm.

The plurality of gate lines G1-Gn of the display panel 300 are connectedto the gate driver 400, and the gate driver 400 alternately applies agate-on voltage Von and a gate-off voltage Voff to the gate lines G1-Gnaccording to the gate control signal CONT1 applied from the signalcontroller 600.

The display panel 300 may be formed by two sheets of substrates whichface each other and are bonded to each other, and the gate driver 400may be formed so as to be attached to one side edge of the display panel300. Further, the gate driver 400 may also be mounted on the displaypanel 300 together with the gate lines G1-Gn, the data lines D1-Dm, andthe switching elements Q. That is, the gate driver 400 may be formedtogether in the process of forming the gate lines G1-Gn, the data linesD1-Dm, and the switching elements Q.

The plurality of data lines D1-Dm of the display panel 300 are connectedto the data driver 500, and the data driver 500 receives the datacontrol signal CONT2 and the image data DAT from the signal controller600. The data driver 500 converts the image data DAT into a data voltageby using a gray voltage generated from a gray voltage generator 800, andtransfers the converted data voltage to the data lines D1-Dm.

Next, a signal controller of the display device according to the firstexemplary embodiment will be described.

FIG. 2 is a block diagram of a signal controller of a display deviceaccording to the first exemplary embodiment.

The signal controller 600 may include a signal receiving unit 610receiving various signals from the graphics processing unit 700, a framememory 640 storing the input image data, and a driving frequencyselecting unit 650 selecting a first frequency when the motion pictureis displayed and selecting a second frequency when the still image isdisplayed.

The signal receiving unit 610 receives the input image data, the stillimage start signal, and the still image end signal from the graphicsprocessing unit 700. Although not shown, the signal receiving unit 610is connected with the graphics processing unit 700 through a main linkand a sub-link. The signal receiving unit 610 receives the input imagedata from the graphic processing unit 700 through the main link.Further, the signal receiving unit 610 receives the still image startsignal and the still image end signal from the graphics processing unit700 through the sub-link and transmits a signal for notifying a drivingstate of the display panel 300 to the graphics processing unit 700.

The frame memory 640 receives and stores the input image data from thesignal receiving unit 610. When the display panel displays the motionpicture, the frame memory 640 is not used. When the display paneldisplays the still image, the input image data is stored in the framememory 640, and the storage image data stored in the frame memory 640 isoutputted to the display panel 300.

The driving frequency selecting unit 650 selects the first frequencywhen the display panel displays the motion picture and selects thesecond frequency when the display panel displays the still image. Whenthe motion picture is displayed, the input image data is received fromthe signal receiving unit 610 to be outputted to the display panel 300at the first frequency. When the still image is displayed, the storageimage data is received from the frame memory 640 to be outputted to thedisplay panel 300 at the second frequency.

In this case, the second frequency has a lower value than the firstfrequency.

For example, the first frequency may be 60 Hz, which means that 60frames are reproduced per second and displayed on the screen. Further,the second frequency may be 10 Hz, which means that 10 frames arereproduced per second and displayed on the screen. In such case, thepower consumption is decreased by ⅙ when displaying the still image ascompared to the display of the moving picture. Accordingly, thefrequency used when displaying the still image is set to be lower thanthe frequency used when displaying the moving picture by a predeterminedratio, and thereby the power consumption is decreased by more than theincrease in the amount of the power consumption as a result of theaddition of the frame memory.

When the motion picture is displayed, if the driving frequency isreduced, there is a problem in that the motion looks unnatural, but whenthe still image is displayed, because the frame having the same imagedata DAT is repeatedly reproduced, although the driving frequency isreduced, such a problem does not occur. However, if the frequency isdecreased, the flicker is increased such that it is preferable todecrease the frequency only to the degree that the flicker does notappear.

Next, a driving method of a display device according to an exemplaryembodiment will be described with reference to FIG. 1 and FIG. 3.

FIG. 3 is a view of control signals of a display device according to thefirst exemplary embodiment.

First, in the first frame as a frame displaying the moving picture, thegraphics processing unit 700 transmits the image data DAT of the movingpicture to the signal controller 600, and the signal controller 600transmits the gate control signal CONT1 to the gate driver 400 and theimage data DAT and the data control signal CONT2 to the data driver 500.At this time, the display panel 300 displays the moving picture with thefirst frequency in the first frame. For example, in the case that thefirst frequency is 60 Hz, the screen is displayed during 1/60 of asecond in the first frame.

That is, the graphics processing unit 700 recognizes the first frame ofthe moving picture and supplies the image data DAT, and the displaypanel 300 displays the moving picture with the first frequency.

Next, in the second frame as a frame displaying the still image, thegraphics processing unit 700 transmits the image data DAT of the stillimage to the signal controller 600 along with a still image start signalinforming signal controller 600 of the start of the still image. Thesignal controller 600 receives the still image start signal to recognizethe start of the still image and stores the image data DAT of the stillimage to the frame memory. Also, the signal controller 600 inactivatesthe graphics processing unit 700 such that the graphics processing unit700 does not transmit the image data DAT of the still image.

The signal controller 600 transmits the image data DAT of the stillimage stored to the frame memory to the data driver 500. Here, thedisplay panel 300 displays the moving picture with the second frequencyin the second frame. For example, in the case that the second frequencyis 40 Hz, the screen is displayed during 1/40 of a second in the secondframe.

That is, in the second frame, the graphics processing unit 700recognizes the second frame of the still image such that it isinactivated, and the display panel 300 displays the still image with thesecond frequency.

Although not shown, the display panel 300 displays the still image withthe second frequency from the third frame to the (n−1)-th frame like itdoes the second frame.

Next, in the n-th frame as a frame corresponding to a position where thestill image is converted into the moving picture, the graphicsprocessing unit 700 transmits the image data DAT of the moving pictureto the signal controller 600 along with a still image finish signalinforming of the finish of the still image.

At this time, the display panel 300 is driven with the second frequencyto the frame before the n-th frame, and the graphics processing unit 700recognizes the display panel 300 that is driven with the firstfrequency, thereby the change of the frequency is generated in themiddle position of the n-th frame. Also, a time delay is generated whilethe image data DAT of the moving picture is transmitted from thegraphics processing unit 700. Accordingly, to prevent deterioration ofthe visibility due as a result of the time delay, the image data DAT ofthe still image is displayed with the second frequency until the n-thframe in which the still image finish signal is applied is finished.

That is, in spite of a period in which the moving picture must bedisplayed to a vertical blank period when the n-th frame is finishedafter the still image finish signal is applied in the middle position ofthe n-th frame, the display panel 300 displays the still image with thesecond frequency.

Next, in the (n+1)-th frame as a frame displaying the moving picture,the graphics processing unit 700 transmits the image data DAT of themoving picture to the signal controller 600, and the display panel 300displays the moving picture with the first frequency.

Next, another method of driving the display device according to thefirst exemplary embodiment will be described with reference to FIG. 1and FIG. 4.

FIG. 4 is a view showing control signals of a display device accordingto the first exemplary embodiment.

The present exemplary embodiment is similar to the above, and thereforeparts that differ from the first exemplary embodiment will be described.

The driving method of the display device in the first frame, the secondframe, and the n-th frame is the same as that described above.

In the (n+1)-th frame, the graphics processing unit 700 recognizes the(n+1)-th frame as the frame displaying the moving picture and transmitsthe signal data DAT of the moving picture to the signal controller 600.

The signal controller 600 transmits the STV signal to the gate driver400 at the start position of each frame, and the gate driver 400 thenreceives the CPV signal from the signal controller 600 to turn on theswitching element Q of the display panel 300. However, the signalcontroller 600 does not transmit the STV signal to the gate driver 400at the start position of the (n+1)-th frame as a position where thesecond frequency is changed into the first frequency. Accordingly,although the CPV signal is applied to the gate driver 400 in the(n+1)-th frame, the STV signal is not applied such that the switchingelement Q of the display panel 300 enters a turned-off state.

That is, the switching element Q is not turned on in the (n+1)-th framesuch that the pixel is not charged and is maintained as the chargedvoltage in the n-th frame, and thereby the display panel 300 displaysthe still image.

Next, in the (n+2)-th frame as a frame displaying the moving picture,the graphics processing unit 700 transmits the image data DAT of themoving picture to the signal controller 600, and the display panel 300displays the moving picture with the first frequency.

The signal controller 600 of the display device according to the firstexemplary embodiment may realize the first frequency and the secondfrequency by various methods.

For example, the various methods include a method of changing a clockfrequency of a gate signal, a method of changing a length of a verticalblank period, a method of changing a clock frequency of a gate signaland changing a length of a vertical blank period at the same time, andthe like, and will be described below with reference to FIGS. 5 to 9.

FIG. 5 is a diagram illustrating a DE signal and a Vsync signal used inthe display device according to the first exemplary embodiment. FIG. 6is a diagram illustrating a gate signal and an STV signal when a displaypanel is driven at a first frequency in the display device according tothe first exemplary embodiment. FIGS. 7 to 9 are diagrams illustrating agate signal and an STV signal when a display panel is driven at a secondfrequency in the display device according to the first exemplaryembodiment.

As shown in FIG. 5, one frame is configured by an effective period inwhich image data is transmitted and a vertical blank period in whichimage data is not transmitted. Image data of two adjacent frames may bedivided by the vertical blank period.

As shown in FIG. 6, when the display panel 300 is driven at the firstfrequency, the gate signal is supplied in the effective period so thatpixel voltage corresponding to the image data may be applied. A gate-offstate may be maintained in the vertical blank period.

As shown in FIG. 7, when the display panel 300 is driven at the secondfrequency, the length of one frame is increased as compared with thetime when the display panel 300 is driven at the first frequency. Forexample, in the case in which the first frequency is 60 Hz and thesecond frequency is 20 Hz, as shown in FIG. 4, when the display panel300 is driven at the second frequency, the length of one frame isincreased by three times as compared with the frame length when thedisplay panel 300 is driven at the first frequency. In this case, thelength of the effective period when the display panel 300 is driven atthe second frequency is three times or more longer than the length ofthe effective period when the display panel 300 is driven at the firstfrequency, thereby implementing the second frequency. In order toincrease the length of the effective period, the clock frequency of thegate signal may be increased about three times or more. When the displaypanel 300 is driven at the first frequency or at the second frequency,the lengths of the vertical blank periods are not significantlydifferent from each other.

FIG. 8 is the same as FIG. 7 in that the length of one frame when thedisplay panel 300 is driven at the second frequency is increased threetimes as compared with the time when the display panel 300 is driven atthe first frequency. Unlike FIG. 7, however, in FIG. 8, the length ofthe effective period when the display panel 300 is driven at the secondfrequency is almost the same as the length of the effective period whenthe display panel 300 is driven at the first frequency. However, thelength of the vertical blank period when the display panel 300 is drivenat the second frequency is increased by as much as a lengthcorresponding to two frames when the display panel 300 is driven at thefirst frequency, thereby implementing the second frequency.

FIG. 9 is the same as FIGS. 7 and 8 in that the length of one frame whenthe display panel 300 is driven at the second frequency is increasedthree times as compared with the time when the display panel 300 isdriven at the first frequency. Unlike FIGS. 7 and 8, however, in FIG. 9,the length of the effective period when the display panel 300 is drivenat the second frequency is longer than the length of the effectiveperiod when the display panel 300 is driven at the first frequency.Simultaneously, the length of the vertical blank period when the displaypanel 300 is driven at the second frequency is longer than the length ofthe effective period when the display panel 300 is driven at the firstfrequency. That is, the length of the effective period when the displaypanel 300 is driven at the second frequency has a length correspondingto about two frames when the display panel 300 is driven at the firstfrequency, and the length of the vertical blank period when the displaypanel 300 is driven at the second frequency has a length correspondingto one frame when the display panel 300 is driven at the firstfrequency, thereby implementing the second frequency.

As described above, when the still image is displayed, a drivingfrequency of the display panel 300 is decreased, such that powerconsumption may be reduced.

If the still image start signal is applied, the signal controller 600controls the signals such that the display panel 300 is driven with thesecond frequency. At this time, the frequency of the display panel 300that was driven with the first frequency may be directly converted intothe second frequency at the position where the still image start signalis applied. Also, after the still image start signal is applied, thedisplay panel 300 may be driven with a frequency that is higher than thesecond frequency and lower than the first frequency during a frame 51.That is, after the still image start signal is applied, the displaypanel 300 may be converted into the second frequency after the passageof the frame 51 through a transient period during the frame S1. Torealize this, during the frame 51 after the still image start signal isapplied, the length of the vertical blank period may be graduallyincreased.

If the still image start signal is applied, the signal controller 600controls the signals such that the display panel 300 is driven with thefirst frequency. The frequency of the display panel 300 that was drivenwith the second frequency may be directly converted into the firstfrequency at the time when the still image start signal is applied.Also, after the still image start signal is applied, the display panel300 may be driven with a frequency that is higher than the secondfrequency and lower than the first frequency during a frame S2. That is,after the still image start signal is applied, the display panel 300 maybe converted into the second frequency after the passage of the frame S2through the transient period during the frame S2. To realize this,during the frame S1 after the still image start signal is applied, thelength of the vertical blank period may be gradually decreased.

As described above, in the display device according to the firstexemplary embodiment, different clock signals are used to drive thestill image and the motion picture with different frequencies. Next, thewidth of the CPV signal according to the usage of the different clocksignals of the still image and the moving picture will be described withreference to FIG. 10 and FIG. 11.

FIG. 10 is a view showing a clock signal and a CPV signal when beingdriven with the first frequency in a display device according to thefirst exemplary embodiment, and FIG. 11 is a view showing a clock signaland a CPV signal when being driven with the second frequency in adisplay device according to an exemplary embodiment. The clock signal isindicated by CLK, and the CPV signal is indicated by CPV.

As shown in FIG. 10, in the display device according to an exemplaryembodiment, the width W3 of the CPV signal when being driven with thefirst frequency corresponds to six clock periods. If the CPV signal isset to have a width corresponding to the six clock periods when thedisplay panel is driven with the second frequency, the clock signal whenbeing driven with the first frequency is different from the clock signalwhen being driven with the second frequency such that the width of theCPV signal is changed.

As shown in FIG. 11, in the display device according to an exemplaryembodiment, the width W4 of the CPV signal when being driven with thesecond frequency has a width corresponding to three clock periods.Accordingly, although the clock speed when being driven with the secondfrequency is later than the clock speed when being driven with the firstfrequency, by differentiating parameters of the widths of the CPVsignals when being driven with the first frequency and the secondfrequency, the widths of the CPV signals may be equally maintained.

That is, for the signal controller, the width W3 of the CPV signal whenthe display panel is driven with the first frequency is set to have thesame width as the p times clock signal (where p is a number), and thewidth W4 of the CPV signal when the display panel is driven with thesecond frequency is set up to have the same width as the q times clocksignal (where q is a number), such that q is less than p. At this time,n and m may be set for the width W3 of the CPV signal when the displaypanel is driven with the first frequency and the width W4 of the CPVsignal when being driven with the second frequency to be equal to eachother.

Accordingly, the change ratio of the pixel when the display paneldisplays the still image is the same as the change ratio of the pixelwhen the display panel displays the moving picture such that anydifference in visibility may be prevented.

Another method of preventing a difference in visibility between thestill image and the moving picture will be described with reference toFIG. 12.

FIG. 12 is a flowchart showing a method of amending image data in adisplay device according to the first exemplary embodiment.

As shown in FIG. 12, the signal controller determines whether thedisplay panel is driven with the first frequency or the second frequencyin the corresponding frame (S110).

At this time, the changing ratios are different in the frames drivenwith the first frequency and the frames driven with the second frequencysuch that the images that are actually displayed are different eventhough the images have the same image data DAT. Accordingly, tocompensate a luminance difference that is generated due to the differentchanging ratios of the pixel in the frames driven with the firstfrequency and the frames driven with the second frequency, gammacorrection is used to compensates the gray characteristics (S120).

Next, the gamma-corrected image data DAT is output in the frame drivenwith the second frequency, and the image data DAT is output without thegamma correction in the frame driven with the first frequency (S130).

That is, the image data DAT in the frame driven with the secondfrequency is gamma-corrected such that any difference in visibility maybe prevented even though the changing ratios of the pixel when thedisplay panel displays the still image and the moving picture differ.

Next, a display device according to the second exemplary embodiment willbe described with reference to FIG. 13 and FIG. 14.

FIG. 13 is a block diagram of a display device according to the secondexemplary embodiment, and FIG. 14 is a block diagram illustrating alight source driver of the display device according to the secondexemplary embodiment.

Because the display device according to the second exemplary embodimentis almost the same as the display device according to the firstexemplary embodiment, just the differences will be described below.

The display device according to the second exemplary embodiment mayfurther include a light source unit 900 irradiating light to the displaypanel 300 and a light source driver 910 controlling signals for drivingthe light source unit 900, as shown in FIG. 13.

The light source unit 900 supplies the light to the inside of thedisplay panel 300 and the supplied light is transmitted to the outsideof the display panel 300 for display on a screen. The light source unit900 may be configured by various light sources, and for example, a lightemitting diode (LED), a cold cathode fluorescent lamp (CCFL), anexternal electrode fluorescent lamp (EEFL), and the like may be used.Further, the light source unit 900 is classified into a side (or edge)light type and a direct light type according to the layout geometrythereof.

The light source driver 910 controls dimming driving of the light sourceunit 900. Dimming driving is a technique for controlling the amount oflight output from the light source in consideration of luminance ofimages, and is used to prevent a contrast ratio (CR) of an image frombeing reduced and also to minimize power consumption.

As shown in FIG. 14, the light source driver 910 includes a drivingfrequency receiving unit 912 receiving the driving frequency of thedisplay panel 300 from the signal controller 600, a light source unitdriving ratio selecting unit 914 determining a driving ratio of thelight source unit 900 according to the driving frequency, and a lightsource driving signal generator 916 generating a signal for driving thelight source 900 according to the driving ratio of the light source unit900.

The driving frequency receiving unit 912 receives the first frequencyfrom the signal controller 600 when the motion picture is displayed, andreceives the second frequency from the signal controller 600 when thestill image is displayed.

The driving ratio selecting unit 914 receives the driving frequency fromthe driving frequency receiving unit 912 to select the ratio for drivingthe light source unit. The driving ratio of the light source unit may bedifferently selected based on the driving frequency.

For example, the driving ratio of the light source unit 900 according tothe driving frequency of the display panel 300 may be selected by usinga look-up table. The driving ratio selecting unit 914 selects thedriving ratio of the light source unit as a first ratio when the drivingfrequency is the first frequency and selects the driving ratio of thelight source unit as a second ratio when the driving frequency is thesecond frequency, by using the look-up table as shown in Table 1. Thatis, when the motion picture is displayed, the light source unit isdriven at the first ratio, and when the still image is displayed, thelight source unit is driven at the second ratio.

TABLE 1 Light source unit Driving frequency (Hz) driving ratio (%) Firstfrequency First ratio Second frequency Second ratio

When the frequency for driving the display panel 300 is changed andreduced, a charging time of each pixel increases and a fully chargedamount increases. Accordingly, luminance may be changed before and aftera point in time when the frequency changes. In a normally black modedisplay device, the luminance increases as the fully charged amountincreases. In a normally white mode display device, the luminancedecreases as the fully charged amount increases.

Accordingly, when the second frequency has a lower value than the firstfrequency, in the normally black mode display device, the second ratiois set to a lower value than the first ratio in order to compensate theincreased luminance. In this case, the power consumption may be reducedby decreasing the driving ratio of the light source unit.

On the contrary, in a normally white mode display device, the secondratio is set to a higher value than the first ratio to compensate forthe decreased luminance.

As described above, the driving ratio of the light source unit 900according to the driving frequency of the display panel 300 may beselected by using a look-up table, but the present disclosure is notlimited thereto, and the driving ratio may be selected by using afunction, y=f(x).

The light source driving signal generator 916 receives the driving ratioof the light source unit selected by the driving ratio selecting unit914 to generate a signal capable of driving the light source unit at thefirst ratio or a signal capable of driving the light source unit at thesecond ratio and transmit the signals to the driver 900. In this case,the signals generated by the light source driving signal generator 916may be various signals such as a PWM signal, a communication protocolsuch as I2C or the like, etc.

Hereinafter, a driving method of the display device according to thesecond exemplary embodiment will be described.

FIG. 15 is a flowchart illustrating a driving method of the displaydevice according to the second exemplary embodiment, FIG. 16 to FIG. 18are block diagrams of the signal controller illustrating the drivingmethod of the display device according to the second exemplaryembodiment for each step in sequence. FIG. 19 and FIG. 20 are blockdiagrams of the light source driver illustrating the driving method ofthe display device according to the first exemplary embodiment for eachstep in sequence.

First, as shown in FIG. 16, the graphics processing unit transmits theinput image data to the signal receiving unit 610 of the signalcontroller 600 (S1110).

It is determined whether or not the still image start signal is appliedto the signal receiving unit (S1120), and if the still image startsignal is not applied, the input image data is outputted to the displaypanel (S1190).

If the still image start signal is applied, as shown in FIG. 17, theinput image data is stored in the frame memory 640 (S1140).

Subsequently, as shown in FIG. 18, the graphics processing unit isinactivated so that the graphics processing unit does not transmit theinput image data, and the storage image data stored in the frame memory640 is output. If the still image start signal is applied, the drivingfrequency selecting unit 650 selects the second frequency to output thestorage image data to the display panel at the second frequency (S1150).In this case, the display panel displays the still image and is drivenat the second frequency.

Simultaneously, as shown in FIG. 19, in the light source driver 910, thedriving frequency receiving unit 912 receives a second frequency f₂ asthe driving frequency and the light source unit driving ratio selectingunit 914 selects a second ratio P₂ as the driving ratio of the lightsource unit.

The driving ratio of the light source unit may be differently selectedaccording to the driving frequency. In this case, the driving ratio ofthe light source unit 900 according to the driving frequency of thedisplay panel may be selected by using the look-up table or a function,y=f(x).

The light source driving signal generator 916 generates a light sourcedriving signal that is capable of driving the light source unit at thesecond ratio P₂ to output the generated light source driving signal tothe light source unit. In this case, the light source driving signal maybe various signals such as a PWM signal, a communication protocol suchas I2C or the like, etc.

Subsequently, it is determined whether or not the still image end signalis applied (S1160), and if the still image end signal is not applied,the storage image data is outputted at the second frequency and thelight source unit is driven at the second ratio (S1150).

If the still image end signal is applied, as shown in FIG. 16, thegraphics processing unit is re-activated so as to transmit the inputimage data (S1180).

If the still image end signal is applied, the driving frequencyselecting unit 650 selects the first frequency to output the input imagedata to the display panel at the first frequency. In this case, thedisplay panel displays the motion picture and is driven at the firstfrequency (S1190).

Simultaneously, as shown in FIG. 20, in the light source driver 910, thedriving frequency receiving unit 912 receives a first frequency f₁ asthe driving frequency and the light source unit driving ratio selectingunit 914 selects the first ratio P₁ as the driving ratio of the lightsource unit.

The light source driving signal generator 916 generates a light sourcedriving signal that is capable of driving the light source unit at thefirst ratio P₁ to output the generated light source driving signal tothe light source unit.

In the driving method of the displaying device according to the secondexemplary embodiment, when the motion picture is displayed, the displaypanel is driven at the first frequency and the light source unit isdriven at the first ratio. Further, when the still image is displayed,the display panel is driven at the second frequency and the light sourceunit is driven at the second ratio.

In this case, the second frequency has a lower value than the firstfrequency. Because the same image is displayed for every frame, thestill image can be implemented even at a low driving frequency. However,a charging time of the pixel is changed according to a change in thedriving frequency, and the fully charged amount is changed. As a result,a change in the luminance may be evident to a viewer's eyes.

Accordingly, the light source unit is dimming-driven, such that anychange in luminance may be imperceptible. In detail, when the displaypanel is driven at the first frequency, the light source unit is drivenat the first ratio, and when the display panel is driven at the secondfrequency, the light source unit is driven at the second ratio.

In a normally black mode display device, the second ratio is set to alower value than the first ratio. In this case, the first ratio and thesecond ratio are set as values that are capable of compensating theluminance increasing when the still image is displayed as compared withthe motion picture.

In the normally white mode display device, the second ratio is set to ahigher value than the first ratio. In this case, the first ratio and thesecond ratio are set as values that are capable of compensating theluminance decreasing when the still image is displayed as compared withthe motion picture.

When a still image changes into a motion picture, the time at which thedriving frequency of the display panel changes and the time at which thedriving ratio of the light source unit is changed coincide with avertical blank period V-blank, such that any luminance change isimperceptible.

Next, a display device according to the third exemplary embodiment willbe described with reference to FIG. 13, FIG. 14, and FIG. 21.

FIG. 21 is a block diagram of a signal controller according to the thirdexemplary embodiment. The third exemplary embodiment is the same as thedisplay device of the second exemplary embodiment except for the signalcontroller, and will be described with reference to FIG. 13 and FIG. 14.

Because the display device according to the third exemplary embodimentis almost the same as the display device according to the secondexemplary embodiment, just differences will be described below. Onesignificant difference between the second exemplary embodiment and thefirst exemplary embodiment is that the signal controller in the secondexemplary embodiment further includes a frame counting unit, and it willbe described in detail.

A display device according to the third exemplary embodiment is the sameas the display device according to the second exemplary embodiment inthat the display device according to the second exemplary embodimentincludes a display panel 300 displaying an image, a signal controller600 controlling signals for driving the display panel 300, a graphicsprocessing unit 700 transmitting input image data to the signalcontroller 600, a light source unit 900 irradiating light to the displaypanel 300, and a light source driver 910 controlling signals for drivingthe light source unit 900, all as shown in FIG. 13.

The signal controller 600 as shown in FIG. 21 may include a signalreceiving unit 610 receiving various signals from the graphicsprocessing unit 700, a frame counting unit 620 counting the number offrames, a frame memory 640 storing the input image data, and a drivingfrequency selecting unit 650 selecting a first frequency when displayingthe motion picture and selecting a second frequency when displaying thestill image.

The signal receiving unit 610 receives the input image data, the stillimage start signal, and the still image end signal from the graphicsprocessing unit 700. Although not shown, the signal receiving unit 610is connected with the graphics processing unit 700 through a main linkand a sub-link. The signal receiving unit 610 receives the input imagedata from the graphics processing unit 700 through the main link.Further, the signal receiving unit 610 receives the still image startsignal and the still image end signal from the graphics processing unit700 through the sub-link, and transmits a signal for notifying a drivingstate of the display panel 300 to the graphic processing unit 700.

The frame counting unit 620 counts the number of still image sequentialframes inputted before the still image end signal is applied after thestill image start signal is applied, and counts the number of motionpicture sequential frames inputted until the still image start signal isapplied after the still image end signal is applied.

The frame counting unit 620 transmits the input image data to the framememory 640 when the number of the still image sequential frames is equalto or more than a value x. Further, the graphics processing unit 700 isinactivated so that the graphics processing unit 700 does not transmitthe input image data. On the contrary, when the number of the stillimage sequential frames is less than x, the input image data is nottransmitted to the frame memory 640, but is transmitted to the drivingfrequency selecting unit 650, so that the input image data is outputted.Further, the graphics processing unit 700 is not inactivated so that theinput image data is continuously transmitted.

This is so as to not convert the motion picture into the still imagewhen the number of the still image sequential frames is less than x.When the still image is displayed for a short time and then convertedinto the motion picture again, an effect of reducing the powerconsumption is not large if the driving frequency is changedaccordingly, such that the luminance change does not occur bymaintaining the driving frequency. Although the light source unit isdimming-driven according to the change in the driving frequency, theluminance change may be perceptible. Accordingly, when the still imageis displayed for a short time, the driving frequency of the displaypanel 300 and the driving ratio of the light source unit 900 are notchanged but are maintained, such that the luminance change does notoccur.

When the number of the motion picture sequential frames is equal to ormore than a value y, the frame counting unit 620 activates the graphicsprocessing unit 700 so that the graphics processing unit 700 transmitsthe input image data. On the contrary, when the number of the motionpicture sequential frames is less than y, the graphics processing unit700 is maintained in the inactivated state.

This is so as to not convert the still image into the motion picturewhen the number of the motion picture sequential frames is less than y.When the motion picture is displayed for a short time and then convertedinto the still image again, an effect of reducing the power consumptionis not large if the driving frequency is accordingly changed, such thatthe luminance change does not occur by maintaining the drivingfrequency. That is, when the motion picture is displayed for a shorttime, the driving frequency of the display panel 300 and the drivingratio of the light source unit 900 are not changed are but maintained,such that the luminance change may not occur.

In this case, the values of x and y may be appropriately selected andset in consideration of the effect of the reduction in the powerconsumption and the visibility problem according to the luminancechange.

The frame memory 640 receives and stores the input image data from theframe counting unit 620 when the number of the still image sequentialframes is equal to or more than x.

The driving frequency selecting unit 650 selects the first frequencywhen the display panel 300 continuously displays the still image by xframes or more and selects the second frequency when the display panel300 continuously displays the motion picture by y frames or more. Thedriving frequency selecting unit 650 outputs the storage image datastored in the frame memory 640 to the display panel 300 at the firstfrequency when the number of the still image sequential frames is equalto or more than x. The driving frequency selecting unit 650 outputs theinput image data to the display panel 300 at the second frequency whenthe number of the motion picture sequential frames is equal to or morethan y.

Accordingly, the light source driver 910 receives the first frequencyfrom the signal controller 600 to drive the light source unit 900 at thefirst ratio when the number of the still image sequential frames isequal to or more than x. The light source driver 910 receives the secondfrequency from the signal controller 600 to drive the light source unit900 at the second ratio when the number of the motion picture sequentialframes is equal to or more than y.

Hereinafter, a driving method of a display device according to a thirdexemplary embodiment will be described below.

FIG. 22 is a flowchart illustrating a driving method of the displaydevice according to the third exemplary embodiment, and FIG. 23 to FIG.26 are block diagrams of the signal controller illustrating the drivingmethod of the display device according to the third exemplary embodimentfor each step in sequence.

Because the driving method of the display device according to the thirdexemplary embodiment is almost the same as the driving method of thedisplay device according to the second exemplary embodiment, justdifferences will be mainly described below.

First, as shown in FIG. 23, the graphics processing unit transmits theinput image data to the signal receiving unit 610 of the signalcontroller 600 (S2110).

It is determined whether or not the still image start signal is appliedto the signal receiving unit 610 (S2120), and if the still image startsignal is not applied, the input image data is output to the displaypanel. In this case, the display panel displays the motion picture andis driven at the second frequency (S2190).

If the still image start signal is applied, the frame counting unit 620counts the number of the still image sequential frames input before thestill image end signal is applied after the still image start signal isapplied. In this case, the frame counting unit 620 determines whether ornot the number of the still image sequential frames is equal to or morethan x (S2130). When the number of the still image sequential frames isless than x, the input image data is outputted to the display panel likethe case where the still image start signal is not applied. In thiscase, the display panel displays the still image and is driven at thefirst frequency (S2190).

If the number of the still image sequential frames is equal to or morethan x, as shown in FIG. 24, the input image data is stored in the framememory 640 (S2140).

Subsequently, as shown in FIG. 25, the graphics processing unit isinactivated so that the graphics processing unit does not transmit theinput image data, and the storage image data stored in the frame memory640 is output. If the number of the still image sequential frames isequal to or more than x, the driving frequency selecting unit 650selects the second frequency to output the storage image data to thedisplay panel at the second frequency (S2150). In this case, the displaypanel displays the still image and is driven at the second frequency.

Simultaneously, the light source driver receives the second frequency asthe driving frequency to drive the light source unit at the secondratio.

The driving ratio of the light source unit may be differently selectedaccording to the driving frequency. In this case, the driving ratio ofthe light source unit according to the driving frequency of the displaypanel may be selected by using the look-up table or a function, y=f(x).

Subsequently, it is determined whether or not the still image end signalis applied (S2160), and if the still image end signal is not applied,the storage image data is output at the second frequency and the lightsource unit is driven at the second ratio. In this case, the displaypanel displays the still image and is driven at the second frequency(S2150).

As shown in FIG. 26, if the still image end signal is applied, the framecounting unit 620 counts the number of the motion picture sequentialframes input before the still image start signal is applied after thestill image end signal is applied. In this case, the frame counting unit620 determines whether or not the number of the motion picturesequential frames is equal to or more than y (S2170). If the number ofthe motion picture sequential frames is less than y, the storage imagedata is output at the first frequency and the light source unit isdriven at the first ratio, like the case where the motion picture startsignal is not applied (S2150).

If the still image end signal is applied but the number of the motionpicture sequential frames is less than y, the graphics processing unitis activated and the input image data is transmitted to the signalreceiving unit 610. However, the display panel displays the still imageby outputting the storage image data and is driven at the secondfrequency.

If the number of the motion picture sequential frames is equal to ormore than y, as shown in FIG. 12, the graphics processing unit isactivated again so as to transmit the input image data (S2180).

If the number of the motion picture sequential frames is equal to ormore than y, the driving frequency selecting unit 650 selects the firstfrequency to output the input image data to the display panel at thefirst frequency. In this case, the display panel displays the motionpicture and is driven at the first frequency (S2190).

Simultaneously, the light source driver receives the first frequency asthe driving frequency to drive the light source unit at the first ratio.

In the driving method of the display device according to the thirdexemplary embodiment, when the still image is continuously displayed byx frames or more, the display panel is driven at the second frequencyand the light source unit is driven at the second ratio. Further, whenthe motion picture is continuously displayed by y frames or more, thedisplay panel is driven at the first frequency and the light source unitis driven at the first ratio.

In this case, the second frequency has a lower value than the firstfrequency. Because the same image is displayed for every frame, thestill image may be implemented even by a low driving frequency. However,a charging time of the pixel is changed according to a change in thedriving frequency and the charged charge amount is changed. The changeof the luminance according thereto may be noticable to a viewer.

Accordingly, the light source unit is dimming-driven, such that thechange in the luminance may not be noticable to a viewer. In detail,when the display panel is driven at the second frequency, the lightsource unit is driven at the second ratio, and when the display panel isdriven at the first frequency, the light source unit is driven at thefirst ratio.

Further, when the still image is not continuously displayed by x framesor more and when the motion picture is not continuously displayed by yframes or more, the driving frequency of the display panel and thedriving ratio of the light source unit are not changed but aremaintained, such that the luminance change does not occur.

When the still image end signal is applied but the number of the motionpicture sequential frames is less than y, the display panel displays thestill image by outputting the storage image data and is driven at thesecond frequency, but the present disclosure is not limited thereto. Onthe contrary, when the still image end signal is applied but the numberof the motion picture sequential frames is less than y, the input imagedata may be outputted at the first frequency and the light source unitmay be driven at the first ratio. In this case, the display paneldisplays the motion picture and is driven at the first frequency.

Next, a driving method of a display device according to the fourthexemplary embodiment will be described with reference to FIG. 27 to FIG.29. A structure of the display device according to the fourth exemplaryembodiment is the same as the structure of the display device accordingto the second exemplary embodiment, and a description thereof isomitted.

FIG. 27 is a flowchart of a driving method of a display device accordingto the fourth exemplary embodiment, and FIG. 28 and FIG. 29 are views ofan STV signal and a light source unit driving ratio of a display deviceaccording to the fourth exemplary embodiment.

First, the graphics processing unit transmits the input image data tothe signal receiving unit 610 of the signal controller 600 (S3110).

It is determined whether or not the still image start signal is appliedto the signal receiving unit (S3120), and if the still image startsignal is not applied, the input image data is outputted to the displaypanel (S3190).

If the still image start signal is applied, the input image data isstored in the frame memory 640 (S3140).

Subsequently, the graphics processing unit is inactivated so that thegraphics processing unit does not transmit the input image data and thestorage image data stored in the frame memory 640 is output. If thestill image start signal is applied, the driving frequency selectingunit 650 selects the second frequency to output the storage image datato the display panel at the second frequency (S3150). In this case, thedisplay panel displays the still image and is driven at the secondfrequency.

Simultaneously, in the light source driver 910, the driving frequencyreceiving unit 912 receives a second frequency f₂ as the drivingfrequency and the light source unit driving ratio selecting unit 914selects a second ratio P₂ as the driving ratio of the light source unit.

Next, a periodic change of the light source unit driving ratio will bedescribed with reference to FIG. 28.

STV1 of FIG. 28 is an STV signal when the display panel is driven withthe first frequency, and STV2 is an STV signal when the display panel isdriven with the second frequency.

Firstly, when the first frequency is 60 Hz and the second frequency is10 Hz, STV2 is applied one time during a time that STV1 is applied sixtimes. Accordingly, the luminance of the screen is frequently changedwhen being driven with the first frequency rather than the secondfrequency, and thereby the flicker is relatively imperceptible.Accordingly, in the display device according to the fourth exemplaryembodiment, the light source unit driving ratio is changed with the samecycle as the application cycle of the STV1 signal when being driven withthe first frequency.

First, at a position where the STV2 is applied, the light source unitdriving ratio selecting unit 914 selects the first ratio with the lightsource unit driving ratio.

The light source driving signal generator 916 generates a light sourcedriving signal that is capable of driving the light source unit at thefirst ratio P1 to output the generated light source driving signal tothe light source unit. In this case, the light source driving signal maybe various signals such as a PWM signal, a communication protocol suchas I2C or the like, etc.

When one frame is divided into the first to the sixth periods having thesame length, the light source unit is driven with the first ratio in thefirst period.

Next, when using the normally black mode display device, at the positionwhere the second period is started, the light source unit driving ratioselecting unit 914 selects the second ratio that is lower than the firstratio as the light source unit driving ratio to drive the light sourceunit with the second ratio.

Next, at the position where the third period is started, the lightsource unit driving ratio selecting unit 914 selects the third ratiothat is lower than the second ratio as the light source unit drivingratio to drive the light source unit with the third ratio.

Next, at the position where the fourth period is started, the lightsource unit driving ratio selecting unit 914 selects the fourth ratiothat is lower than the third ratio as the light source unit drivingratio to drive the light source unit with the fourth ratio.

Next, at the position where the fifth period is started, the lightsource unit driving ratio selecting unit 914 selects the fifth ratiothat is lower than the fourth ratio as the light source unit drivingratio to drive the light source unit with the fifth ratio.

Next, at the position where the sixth period is started, the lightsource unit driving ratio selecting unit 914 selects the sixth ratiothat is lower than the fifth ratio as the light source unit drivingratio to drive the light source unit with the sixth ratio.

Next, at the position where the next period is started, the light sourceunit driving ratio selecting unit 914 again selects the first ratio asthe light source unit driving ratio to drive the light source unit withthe first ratio.

That is, the light source unit is driven with the first ratio or theratio that is sequentially decreased from the first ratio. At the timewhen the STV2 signal is transmitted, the light source unit is drivenwith the first ratio, and before the transmission of the next STV2signal, the light source unit is driven with the ratio that sequentiallydecreases in each period from the first ratio. The change cycle of thelight source unit driving ratio may be set to be the same as thetransmission cycle of the STV1 signal. Accordingly, although the displaypanel is driven with the second frequency that is lower than the firstfrequency, the change cycle of the luminance is increased like thedriving with the first frequency such that the flicker is not noticableby a viewer.

Subsequently, it is determined whether or not the still image end signalis applied (S3160), and if the still image end signal is not applied,the storage image data is output at the second frequency and the lightsource unit is driven at the ratio that is periodically changed (S3150).

If the still image end signal is applied, the graphics processing unitis again activated so as to transmit the input image data (S3180).

If the still image end signal is applied, the driving frequencyselecting unit 650 selects the first frequency to output the input imagedata to the display panel at the first frequency. In this case, thedisplay panel displays the motion picture and is driven at the firstfrequency (S3190).

Simultaneously, as shown in FIG. 20, in the light source driver 910, thedriving frequency receiving unit 912 receives a first frequency as thedriving frequency and the light source unit driving ratio selecting unit914 selects the first ratio as the driving ratio of the light sourceunit.

The light source driving signal generator 916 generates a light sourcedriving signal capable of driving the light source unit at the firstratio to output the generated light source driving signal to the lightsource unit.

In the driving method of the displaying device according to the fourthexemplary embodiment, when the motion image is displayed, the displaypanel is driven at the first frequency and the light source unit isdriven at the first ratio. Further, when the still picture is displayed,the display panel is driven at the second frequency and the light sourceunit is driven at the second ratio.

In the above example, the case of using the normally black mode displaydevice was described. The periodic change of the light source unitdriving ratio of the case using the normally white mode display devicewill now be described with reference to FIG. 29.

At the position where the STV2 is applied, the light source unit drivingratio selecting unit 914 selects the first ratio as the light sourceunit driving ratio to drive the light source unit with the first ratio.

Next, at the time where the second period is started, the light sourceunit driving ratio selecting unit 914 selects the second ratio that ishigher than the first ratio as the light source unit driving ratio todrive the light source unit with the second ratio.

Next, in the third period to the sixth period, the light source unit isdriven with the ratio that is gradually increased from the second ratio.

That is, when driving the display panel with the second frequency, inthe example of the normally black mode display device, the light sourceunit is driven with the first ratio and the ratio that is sequentiallydecreased from the first ratio in one frame. In contrast, in the exampleof the normally white mode display device, the light source unit isdriven with the first ratio and the ratio that is sequentially increasedfrom the first ratio in one frame.

Next, a display device according to the fifth exemplary embodiment willbe described with reference to FIG. 30 as well as FIG. 1 and FIG. 2.

FIG. 30 is an equivalent circuit diagram of one pixel of a displaydevice according to the fifth exemplary embodiment.

The display device according to the fifth exemplary embodiment, as shownin FIG. 1, includes the display panel 300, the signal controller 600,the graphics processing unit 700, and the signal controller 600, and asshown in FIG. 2, includes the signal receiving unit 610, the framememory 640, and the driving frequency selecting unit 650.

The display panel 300, the signal controller 600, and the graphicsprocessing unit 700 of the display device according to the fifthexemplary embodiment are the same as those of the first exemplaryembodiment such that the detailed description is omitted.

In the display panel of the display device according to the fifthexemplary embodiment, as shown in FIG. 30, the gate line G and the dataline D are crossed to define the pixel. Although not shown by omitting alayout view and a cross-sectional view, the gate line G and the dataline D may be formed on a substrate and formed on different layers so asto be separated from each other. As shown in FIG. 1, the gate line G andthe data line D may be in plural, but in FIG. 30, because only one pixelis shown, just one gate line G and one data line D are shown.

A switching element Q1 is formed so as to be connected with the gateline G and the data line D. The first switching element Q1 is athree-terminal element such as a thin film transistor and the like, acontrol terminal thereof is connected with the gate line G, an inputterminal thereof is connected with the data line D, and an outputterminal thereof is connected with a liquid crystal capacitor Clc and astorage capacitor Cst.

A storage electrode line SL and a storage electrode control line SCL maybe further formed on the substrate, and the storage electrode line SLand the storage capacitor Cst are connected to each other by a secondswitching element Q2 and a third switching element Q3. That is, thesecond switching element Q2 and the third switching element Q3 areformed between the storage electrode line SL and the storage capacitorCst.

The second switching element Q2 is a three-terminal element such as athin film transistor and the like, a control terminal thereof isconnected with the gate line G, an input terminal thereof is connectedwith the storage electrode line SL, and an output terminal thereof isconnected with the storage capacitor Cst.

The third switching element Q3 is a three-terminal element such as athin film transistor and the like, a control terminal thereof isconnected with the storage electrode control line SCL, an input terminalthereof is connected with the storage electrode line SL, and an outputterminal thereof is connected with the storage capacitor Cst.

Hereinafter, a voltage relationship when the still image is displayed onthe display panel of the display device according to an exemplaryembodiment will be described below.

FIG. 31 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe fifth exemplary embodiment.

In the display device according to the fifth exemplary embodiment, whenthe motion picture is displayed, the display panel is driven at a firstfrequency, and when the still image is displayed, the display panel isdriven at the second frequency that is lower than the first frequency.In this case, in order to implement the second frequency, a length of avertical blank period may be increased as compared with the case wherethe display panel is driven at the first frequency.

For example, in order to change the driving frequency from 60 Hz to 10Hz, the length of the vertical blank period between two adjacenteffective periods may be changed so as to be five times that of oneframe. In this case, speeds for applying a data enable signal DE in boththe driving at 60 Hz and the driving at 10 Hz are the same.

When the still image is displayed by driving the display panel at thesecond frequency, first, if the gate-on voltage is applied to the gateline G in an effective period of the n-th frame, the first switchingelement Q1 and the second switching element Q2 are turned on. Next, if adata voltage is applied to the data line D, the liquid crystal capacitorClc and the storage capacitor Cst are charged through the firstswitching element Q1.

In this case, one terminal of the storage capacitor Cst is connectedwith the first switching element Q1 to represent the data voltage andthe other terminal thereof is connected with the second switchingelement Q2 to represent the common voltage VSL applied to the storageelectrode line SL. For the n-th frame in which the data enable signal isapplied, the common voltage VSL has a constant value.

After the data voltage is applied to each pixel, the gate-off voltage isapplied to the gate line G and the first switching element Q1 and thesecond switching element Q2 are turned off. Subsequently, the verticalblank period starts and the gate-on voltage is applied to the storageelectrode control line SCL. Accordingly, the third switching element Q3connected to the storage electrode control line SCL is turned on and thecommon voltage is applied from the storage electrode line SL.

The common voltage V_(SL) of the vertical blank period has a highervoltage than the common voltage VSL of the effective period. When thecommon voltage VSL of the effective period has the first voltage, afterthe vertical blank period starts, the common voltage VSL is changed tothe second voltage that is higher than the first voltage. Thereafter,the common voltage VSL has the third voltage that is higher than thesecond voltage after a predetermined time elapses in the vertical blankperiod. The time for applying the second voltage and the time forapplying the third voltage to the storage electrode line SL may be setto be the same.

The times when the common voltage VSL is changed from the first voltageto the second voltage and changed from the second voltage to the thirdvoltage may be set so as to coincide with a time when the voltage of oneterminal of the storage capacitor Cst is discharged, such that a pixelvoltage may be changed from the originally applied data voltage toanother voltage which is lower than the originally applied data voltage.

Subsequently, the vertical blank period ends and the gate-off voltage isapplied to the storage electrode control line SCL. Accordingly, thethird switching element Q3 connected to the storage electrode controlline SCL is turned on.

Simultaneously, the n+1 frame starts and the gate-on voltage is appliedto the gate line G. Accordingly, the first switching element Q1 and thesecond switching element Q2 are turned on. Subsequently, the datavoltage is applied to the data line D, and the liquid crystal capacitorClc and the storage capacitor Cst are charged. In this case, because thestill image is displayed, the data voltages of the n-th frame and the(n+1)-th frame are the same.

When the (n+1)-th frame starts, the common voltage VSL applied to thestorage electrode line SL drops to the first voltage again and istransferred to the other terminal of the storage capacitor Cst throughthe second switching element.

As described above, the common voltage VSL applied to the storageelectrode line SL has a value that is changed when the display panel isdriven at the second frequency. That is, the common voltage VSL has thefirst voltage in an effective periods of the n-th frame and the (n+1)-thframe, the second voltage that is higher than the first voltage and thethird voltage higher than the second voltage sequentially in thevertical blank period between the effective periods of the n-th frameand the (n+1)-th frame. For example, the first voltage may be set to 7.5V, the second voltage may be set to 7.6 V, and the third voltage may beset to 7.7 V.

The voltage of the other terminal of the storage capacitor Cst connectedwith the second switching element Q2 and the third switching element Q3is changed according to the change of the common voltage VSL. Further,the voltage of one terminal of the storage capacitor Cst connected withthe first switching element Q1 is also changed.

For example, when the voltage of one terminal of the storage capacitorCst connected with the first switching element Q1 is 10.5 V in the n-thframe by applying the data voltage, the first switching element Q1 isturned off and the predetermined time elapses, such that the voltagedrops.

When the voltage of one terminal of the storage capacitor Cst drops toabout 10.4 V in the vertical blank period and the third switchingelement Q3 is turned on, the common voltage VSL of 7.6 V is applied tothe other terminal of the storage capacitor Cst. In this case, thevoltage of one terminal of the storage capacitor Cst also increases bythe increase in the voltage of the other terminal of the storagecapacitor Cst to be 10.5 V again.

When the predetermined time elapses and the voltage of one terminal ofthe storage capacitor Cst drops to about 10.4 V, the common voltage VSLapplied to the other terminal of the storage capacitor Cst may increaseto 7.7 V. In this case, the voltage of one terminal of the storagecapacitor Cst also increases by the increase in the voltage of the otherterminal of the storage capacitor Cst to be 10.5 V again.

Thereafter, when the (n+1)-th frame starts, the same data voltage as thedata voltage in the n-th frame is applied to one terminal of the storagecapacitor Cst.

As described above, the voltage of one terminal of the storage capacitorCst may be changed in the vertical blank period through the change ofthe common voltage VSL, such that luminance is changed.

When the motion picture is displayed, the display panel is driven at afrequency that is relatively faster than when the still image isdisplayed, such that flicker is not noticable because a cycle of aluminance change is short. When the still image is displayed, thedisplay panel is driven at a relatively slower frequency than when themotion picture is displayed, such that the flicker is clearly noticable.In the fifth exemplary embodiment, the luminance change is inducedthrough the change in the common voltage VSL, such that the flicker maynot be noticable like the case where the motion picture is displayed.

In the fifth exemplary embodiment, when the motion picture is displayed,since the flicker is not clearly noticable even without the induction ofthe luminance change, the common voltage VSL having a constant value issupplied to the storage electrode line SL.

As described above, when the still image is displayed, that is, when thedisplay panel is driven at the second frequency, the common voltage VSLis changed from the first voltage to the second voltage and from thesecond voltage to the third voltage and returns to the first voltageagain. However, the present disclosure is not limited thereto, and thechange in the common voltage VSL may be implemented by various methods.

For example, the common voltage VSL may be changed as shown in FIG. 32.

FIG. 32 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe fifth exemplary embodiment.

When the display panel is driven at the second frequency, the commonvoltage VSL may have the first voltage in effective periods of the n-thframe and the (n+1)-th frame, and the second voltage that is higher thanthe first voltage in the vertical blank period between effective periodsof the n-th frame and the (n+1)-th frame. That is, the common voltageVSL may increase from the first voltage to the second voltage and thenbe maintained at the second voltage, and may drop to the first voltageagain when the next frame starts.

Further, unlike what is shown in FIG. 32, when the display panel isdriven at the second frequency, the common voltage VSL may have thefirst voltage in effective periods of the n-th frame and the (n+1)-thframe, and the second voltage that is higher than the first voltage inthe vertical blank period between the effective periods of the n-thframe and the (n+1)-th frame. Subsequently, after a predetermined timeelapses in the vertical blank period, the common voltage VSL may havethe fourth voltage that is higher than the third voltage. That is, whenthe common voltage VSL is changed from the first voltage to the secondvoltage, from the second voltage to the third voltage, and from thethird voltage to the fourth voltage, and when the next frame starts, thecommon voltage VSL may drop to the first voltage.

When the vertical blank period is relatively short, although the numberof voltage changes is set to be small, the flicker is not readilyperceptible. On the contrary, when the vertical blank period isrelatively long, the flicker is more readily perceptible, such that thenumber of voltage changes may be set to be larger.

Next, a display device according to the sixth exemplary embodiment willbe described with reference to FIG. 33.

FIG. 33 is an equivalent circuit diagram of one pixel of a displaydevice according to the sixth exemplary embodiment.

The display device according to the sixth exemplary embodiment is thesame as most of the display device according to the fifth exemplaryembodiment, and just differences will be described.

The display device according to the sixth exemplary embodiment does notinclude the second switching element, the third switching element, andthe storage electrode control line, as opposed to the fifth exemplaryembodiment. Also, a shape of the common voltage applied to the storageelectrode line is different from the fifth exemplary embodiment.

In the display panel of the display device according to the sixthexemplary embodiment, as shown in FIG. 33, the gate line G and the dataline D are crossed to define the pixel. The gate line G and the dataline D may be in plural and the pixel may be in plural, but only onepixel is shown in FIG. 33.

A switching element Q1 is formed so as to connect the gate line G andthe data line D. The first switching element Q1 is a three-terminalelement such as a thin film transistor or the like. The control terminalthereof is connected to the gate line G, while the input terminalthereof is connected to the data line D, and an output terminal thereofis connected with a liquid crystal capacitor Clc and a storage capacitorCst.

A storage electrode line SL may be further formed, and the storageelectrode line SL and the storage capacitor Cst are connected to eachother.

That is, in the fifth exemplary embodiment described above, the storageelectrode line SL and the storage capacitor Cst are connected to eachother by the switching element. In contrast, in the present exemplaryembodiment, the storage electrode line SL and the storage capacitor Cstare directly connected to each other.

Hereinafter, a voltage relationship when the still image is displayed onthe display panel of the display device according to another exemplaryembodiment will be described below.

FIG. 34 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe sixth exemplary embodiment.

In the display device according to the sixth exemplary embodiment, whenthe motion picture is displayed, the display panel is driven at a firstfrequency, and when the still image is displayed, the display panel isdriven at a second frequency that is lower than the first frequency. Inthis case, in order to implement the second frequency, the length of avertical blank period may be increased as compared with the case wherethe display panel is driven at the first frequency.

When the still image is displayed by driving the display panel at thesecond frequency, first, if the gate-on voltage is applied to the gateline G in the n-th frame, the first switching element Q1 is turned on.Next, if data voltage is applied to the data line D, the liquid crystalcapacitor Clc and the storage capacitor Cst are charged through thefirst switching element Q1.

In this case, one terminal of the storage capacitor Cst is connectedwith the first switching element Q1 to represent the data voltage, andthe other terminal thereof is connected with the storage electrode lineSL to represent common voltage VSL applied to the storage electrode lineSL. For the n-th frame in which the data enable signal is applied, thecommon voltage VSL has a constant value.

After the data voltage is applied to each pixel, the gate-off voltage isapplied to the gate line G and the first switching element Q1 is turnedoff. Subsequently, the vertical blank period starts and the commonvoltage V_(SL) is changed. The common voltage V_(SL) of the verticalblank period swings into the voltage that is higher than the commonvoltage V_(SL) of the effective period and the same voltage as thecommon voltage V_(SL) in the effective period.

When the common voltage VSL in the effective period is the first voltageand the vertical blank period starts, the common voltage VSL may bechanged to the second voltage that is higher than the first voltage andthen may drop to the first voltage again. Thereafter, after apredetermined time elapses in the vertical blank period, the commonvoltage VSL may be again changed to the second voltage again and maythen drop to the first voltage again. In the vertical blank period, atime when the common voltage VSL has the second voltage may be set to beshorter than a time when the common voltage VSL has the first voltage.

A time when the common voltage VSL is changed from the first voltage tothe second voltage may be set so as to coincide with a time when thevoltage of one terminal of the storage capacitor Cst is discharged so asto be different from the originally applied data voltage by apredetermined voltage or more.

In FIG. 34, the number of times the common voltage VSL is changed fromthe first voltage to the second voltage in the vertical blank periodbetween two adjacent frames and then returns to the first voltage againis two. However, the present disclosure is not limited thereto, and thenumber of times may be variously set. For example, the number of timeswhere the common voltage VSL is changed from the first voltage to thesecond voltage in the vertical blank period between two adjacent framesand then returns to the first voltage again may be set to be only onetime or set to be three times, four times, or the like.

When the vertical blank period is relatively short, although the numberof voltage changes is set to be small, the flicker is not readilyperceptible. On the contrary, when the vertical blank period isrelatively long, the flicker is more readily perceptible, such that thenumber of the voltage changes may be set to be larger.

Subsequently, the vertical blank period ends and the common voltage VSLapplied to the storage electrode line SL is constantly maintained at thefirst voltage.

Simultaneously, the (n+1)-th frame starts and the gate-on voltage isapplied to the gate line G. Accordingly, the first switching element Q1is turned on. Subsequently, the data voltage is applied to the data lineD, and the liquid crystal capacitor Clc and the storage capacitor Cstare charged. In this case, because the still image is displayed, thedata voltages of the n-th frame and the (n+1)-th frame are the same.

As described above, the common voltage VSL applied to the storageelectrode line SL has a value that is changed when the display panel isdriven at the second frequency. That is, the common voltage VSL has thefirst voltage between effective periods of the n-th frame and the(n+1)-th frame and swings at the first voltage and the second voltagethat is higher than the first voltage in the vertical blank periodbetween the effective periods of the n-th frame and the (n+1)-th frame.For example, the first voltage may be set to 7.5 V and the secondvoltage may be set to 7.6 V.

The voltage of the other terminal of the storage capacitor Cst connectedwith the storage electrode line SL is changed according to the change ofthe common voltage VSL. Further, the voltage of one terminal of thestorage capacitor Cst connected with the first switching element Q1 isalso changed.

For example, when the voltage of one terminal of the storage capacitorCst connected with the first switching element Q1 is 10.5 V in the n-thframe by applying the data voltage, the first switching element Q1 isturned off and the predetermined time elapses such that the voltagedrops to 10.4 V.

When the common voltage VSL input to the storage electrode line SLincreases from 7.5 V to 7.6 V, the voltage of the other terminal of thestorage capacitor Cst increases to 7.6 V. In this case, the voltage ofone terminal of the storage capacitor Cst also increases by the increasein the voltage of the other terminal of the storage capacitor Cst andbecomes 10.5 V again. Subsequently, the common voltage VSL drops to 7.5V again.

When the predetermined time elapses and the voltage of one terminal ofthe storage capacitor Cst drops to about 10.4 V, the common voltage VSLapplied to the other terminal of the storage capacitor Cst may increasefrom 7.5 V to 7.6 V. In this case, the voltage of one terminal of thestorage capacitor Cst also increases by the increase in the voltage ofthe other terminal of the storage capacitor Cst and becomes 10.5 Vagain.

Thereafter, when the (n+1)-th frame starts, the same data voltage as thedata voltage in the n-th frame is applied to one terminal of the storagecapacitor Cst.

As described above, the voltage of one terminal of the storage capacitorCst may be changed through the change in the common voltage VSL in thevertical blank period, and accordingly, a cycle of the luminance changeis shortened such that the flicker may not be perceptible.

As described above, the common voltage VSL instantaneously increasesfrom the first voltage to the second voltage and then instantaneouslydecreases from the second voltage to the first voltage again after thepredetermined time elapses. However, the present disclosure is notlimited thereto, and change forms of the common voltage VSL may beimplemented by various methods.

For example, as shown in FIG. 35, the common voltage VSL may be changed.

FIG. 35 is a diagram illustrating each of control signals when a stillimage is displayed on a display panel of a display device according tothe sixth exemplary embodiment.

When the display panel is driven at the second frequency, the commonvoltage VSL may have the first voltage in the effective period of then-th frame and the effective period of the (n+1)-th frame and swing atthe first voltage and the second voltage that is higher than the firstvoltage in the vertical blank period between the effective periods ofthe n-th frame and the (n+1)-th frame. In this case, when the commonvoltage VSL is changed from the first voltage to the second voltage, thecommon voltage VSL may be gradually changed while having a value betweenthe first voltage and the second voltage. Further, even when the commonvoltage VSL is changed from the second voltage to the first voltage, thecommon voltage VSL may be gradually changed while having a value betweenthe first voltage and the second voltage.

Unlike what is shown in FIG. 35, when the common voltage VSL is changedfrom the first voltage to the second voltage, the common voltage VSL maybe gradually changed while having a value between the first voltage andthe second voltage, and when the common voltage VSL is changed from thesecond voltage to the first voltage, the common voltage VSL mayinstantaneously drop. Further, when the common voltage VSL is changedfrom the first voltage to the second voltage, the common voltage VSL mayinstantaneously increase, and when the common voltage VSL is changedfrom the second voltage to the first voltage, the common voltage VSL maygradually change while having a value between the first voltage and thesecond voltage.

Hereinafter, reduction of power consumption in the display deviceaccording to the fifth and sixth exemplary embodiments will bedescribed.

FIG. 36 is a graph illustrating power consumption according to drivingfrequency. In detail, for when the power consumption in the driving of60 Hz is 100% and five different screens are driven at 60 Hz to 10 Hz, aratio of relative power consumption to the power consumption in thedriving of 60 Hz is shown. Further, an average for the ratios of thepower consumption of five different screens is also shown. In the fivedifferent screens, the first screen is a white screen, the second screenis a black screen, the third screen and the fourth screen are screensdisplaying different colors by dividing the entire area into a pluralityof regions, and the fifth screen is Windows® wallpaper.

Since the power consumption when the display panel is driven at 10 Hz isabout 60%, the power consumption is reduced by about 40% as comparedwith the case where the display panel is driven at 60 Hz. Accordingly,the driving frequency when the still image is displayed is set to apredetermined ratio or less as compared with the driving frequency whenthe motion picture is displayed, thereby reducing the power consumptionto the increased power consumption or more according to the addition ofthe frame memory.

When the motion picture is displayed, if the driving frequency isreduced, there is a problem in that the motion looks unnatural. When thestill image is displayed, however, because frames having the same imagedata are repeatedly reproduced, although the driving frequency isreduced, the problem does not occur.

When the display panel is driven at a low driving frequency, however,the flicker is easily perceived. Hereinafter, the appearance of flickeraccording to the fifth exemplary embodiment will be compared with therelated art.

FIG. 37 is a graph illustrating voltage of one terminal of a storagecapacitor when a conventional display panel is driven at 60 Hz. And FIG.38 is a graph illustrating voltage of one terminal of a storagecapacitor when a known display panel is driven at 10 Hz, and FIG. 39 isa graph illustrating voltage of one terminal of a storage capacitor whena display panel according to the fifth exemplary embodiment is driven at10 Hz.

By comparing FIG. 37 and FIG. 38, when the display panel is driven at 10Hz, a cycle of the voltage change in one terminal of the storagecapacitor is lengthened as compared with the case where the displaypanel is driven at 60 Hz, such that a cycle of the luminance change islengthened. Accordingly, as the driving frequency becomes lower, theflicker is more easily perceived. Referring to FIG. 39, in the exemplaryembodiment, when the still image is displayed at the low drivingfrequency, the common voltage is changed and the cycle of the voltagechange in one terminal of the storage capacitor may be decreased to thelevel when the display panel is driven at 60 Hz. Accordingly, the cycleof the luminance change is shortened, such that the flicker is notperceptible.

Next, a display device according to the seventh exemplary embodimentwill be described with reference to FIG. 40 as well as FIG. 1.

FIG. 40 is a block diagram of a signal controller of a display deviceaccording to the seventh exemplary embodiment.

The display device according to the seventh exemplary embodimentincludes the display panel 300, the signal controller 600, and thegraphics processing unit 700, as shown in FIG. 1.

The display panel 300 and the graphics processing unit 700 of thedisplay device according to the seventh exemplary embodiment are thesame as in the first exemplary embodiment such that the detaileddescription thereof is omitted.

The signal controller 600 of the display device according to the seventhexemplary embodiment may include a frame memory 640 storing the inputimage data, a calculator 625 calculating a representative value of thestorage image data stored in the frame memory, a line memory 630 storingthe representative value, and a kick-back corrector 660 generatingauxiliary image data by correcting the representative value, as shown inFIG. 40.

The frame memory 640 stores the input image data transmitted from thegraphics processing unit 700. The frame memory 640 is not used when thedisplay panel displays the motion picture, but is used when the displaypanel displays the still image. When the still image start signal isapplied, the input image data is stored in the frame memory 640 and thedisplay panel 300 is driven by using the storage image data stored inthe frame memory 640.

The calculator 625 receives the storage image data from the frame memory640 to calculate the representative value representing the storage imagedata. In this case, the representative value is calculated for each ofthe data lines D1 to Dm.

The storage image data capable of displaying one frame is stored in theframe memory 640, and the storage image data is divided for each of thedata lines D1 to Dm. For example, the storage image data are dividedinto storage image data corresponding to data voltage to be applied to afirst data line D1, storage image data corresponding to data voltage tobe applied to a second data line D2, storage image data corresponding todata voltage to be applied to a third data line D3, and storage imagedata corresponding to data voltage to be applied to an m-th data lineDm.

The calculator 625 receives the storage image data for each of the datalines D1 to Dm to calculate the representative value representing thestored image d. For example, the calculator 625 calculates a firstrepresentative value representing the storage image data correspondingto the data voltage to be applied to the first data line D1, andcalculates a second representative value representing the storage imagedata corresponding to the data voltage to be applied to the second dataline D2. By this method, a third representative value, an m-threpresentative value, and the like are calculated.

The representative value representing the storage image data may becalculated in various methods.

Hereinafter, a method of calculating the representative value will bedescribed below with reference to Table 2.

Table 2 shows a gray value of the storage image data corresponding tothe data voltage to be applied to the first data line D1. The number ofstorage image data corresponding to data voltage applied to one of thedata lines D1 to Dm is the same as the number of the gate lines G1 toGn.

TABLE 2 Storage image data Gray value d11 00100110 d12 00101010 d1300111101 d14 00111011 d15 00111011 d16 00101101 d17 00110001

By a first method, an average gray value of the storage image data maybe set as the representative value and calculated according to Equation1.

$\begin{matrix}{{Gr} = {\sum\limits_{p = 1}^{n}\;\frac{dlp}{n}}} & ( {{Equation}\mspace{14mu} 1} )\end{matrix}$

(Gr: representative value, n: the number of storage image data)

In Table 2, when the average gray value is calculated on the assumptionthat n is 7, the average gray value is 00110010.

By a second method, an average gray value of upper t bits of the storageimage data may be set as the representative value. In this case, a valuefor t may be variously set, and as t is smaller, the calculation isfurther simplified. For example, the value for t may be set to 3 or 4when the storage image data is 8 bits.

Upper 4 bits of the storage image data are extracted below on theassumption that t is 4. d11, d12, d13, d14, d15, d16, and d17 have upper4-bit gray values such as 0010, 0010, 0011, 0011, 0011, 0010, and 0011.The average value thereof is 0011 and the representative value is00110000.

By a third method, a middle value of a maximum gray value and a minimumgray value of the storage image data may be set as the representativevalue. In Table 2, the maximum gray value of the storage image data is00111101 and the minimum gray value is 00100110. The calculated middlevalue thereof is 00110010.

The representative values calculated by the three methods are 00110010,00110000, and 00110010, respectively, and when the representative valuesare expressed by decimals, the decimals are 50, 48, and 50,respectively. Therefore, the representative values are not largelydifferent from each other in spite of following any method. The mostappropriate representative value may be calculated according to thefirst method, but the calculation is complicated. Further, thecalculation is simplified according to the second and third methods,while the representative values in the second and third methods may haverelatively lower appropriateness than the first method.

The line memory 630 receives and stores the representative value fromthe calculator 625. In this case, since the representative value isprovided for each data line, the representative value is stored for eachdata line. Each of the first representative value, the secondrepresentative value, the third representative value, the m-threpresentative value, and the like is stored.

The kick-back corrector 660 corrects the representative value stored inthe line memory 630 according to a kick-back voltage to generate theauxiliary image data.

The data voltage applied from the data lines D1 to Dm is charged in eachpixel connected to the gate lines G1 to Gn and the data lines D1 to Dm,and the charged voltage is referred to as pixel voltage. The pixelvoltage is reduced by parasitic capacitance and the like while theswitching element Q is turned off, and in this case, the reduced voltageis referred to as a kick-back voltage.

The kick-back corrector 660 generates auxiliary image data having avalue most approximate to a gray value corresponding to pixel voltagecharged in a pixel array connected to one of the data lines D1 to Dmwhen the switching element Q is turned off. That is, the auxiliary imagedata has a value approximate to a gray value corresponding to pixelvoltage which is reduced by the kick-back voltage.

The kick-back voltage depends on the magnitude of the data voltageapplied to a corresponding pixel. That is, the kick-back voltage dependson the gray value of the image data corresponding to the data voltage,and may be verified through FIG. 41.

FIG. 41 is a graph illustrating kick-back voltage depending on a grayvalue of image data.

Referring to FIG. 41, as the gray value of the image data becomeslarger, the kick-back voltage is higher. For example, the kick-backvoltage got a gray of 0 is approximately 1.0 V, and the kick-backvoltage for a gray of 256 is approximately 1.2 V. The kick-back voltagedepending on the gray value of the image data shown in FIG. 41 is justexemplified, and is a value which depends on the specification of thedisplay device.

The kick-back voltage is differentiated according to the gray value ofthe image data, but the difference is not large. Therefore, the voltagefor correction depending on the kick-back voltage may be set as the samevoltage. For example, it may be assumed that the kick-back voltage is 1V regardless of the size of the image data.

However, although it is assumed that the kick-back voltage is 1 Vregardless of the size of the image data, the gray value correspondingto 1 V depends on the gray value of each image data. The reason thereforis that voltage and transmittance have a non-linear relationship.Accordingly, the gray value corresponding to the kick-back voltage, thatis, a kick-back correction gray value according to the gray value of theimage data, may be acquired from a voltage-transmittance curve (V-Tcurve) of each display device.

Hereinafter, a method of acquiring the kick-back correction gray valuewill be described with reference to FIG. 42.

FIG. 42 is a graph illustrating a kick-back correction gray valuedepending on the gray value of the image data. The dotted linerepresents a calculation value acquired by calculation, and the solidline represents an approximate value generated by using a calculationvalue.

First, a method of acquiring the kick-back correction gray value by thecalculation will be described below.

Second image data corresponding to second data voltage acquired bysubtracting the kick-back voltage from the first data voltagecorresponding to predetermined first image data is acquired. A valueacquired by subtracting a gray value of the second image data from agray value of the first image data is the kick-back correction grayvalue. By such a method, the kick-back correction gray values for allthe first image data may be acquired and may be expressed in a look-uptable. Further, when the kick-back correction gray values acquired bythe calculation are expressed in the graph, the kick-back correctiongray values are marked with dotted lines of FIG. 42.

The kick-back correction gray values depending on the representativevalue of the storage image data may be acquired by using the look-uptable prepared by the calculation.

Subsequently, a method of acquiring the kick-back correction gray valuethrough approximation by using a calculation value will be describedbelow.

Referring to FIG. 42, when the image data is approximately a gray of175, the kick-back correction gray value is the largest. Further, whenthe image data is in a range smaller than approximately a gray of 175,as the gray value becomes smaller, the magnitude of the kick-backcorrection gray value becomes smaller, and when the image data is in arange larger than approximately a gray of 175, as the gray value becomeslarger, the magnitude of the kick-back correction gray value becomessmaller. In this case, variation in the kick-back correction gray valuedepending on the gray value of the image data shows non-linearity, butthe variation has a pattern close to linearity.

Therefore, a function of the kick-back correction gray value dependingon the gray value of the image data may be generated by using linearinterpolation. In this case, a function of Equation 2 may be generatedby using a kick-back correction gray value y1 at a minimum gray x1, akick-back correction gray value x3 at a maximum gray x2, and a grayvalue y2 when the magnitude of the kick-back correction gray value is amaximum of y2.

$\begin{matrix}{y = \begin{matrix}{{\frac{{y\; 1} - {y\; 2}}{{x\; 1} - {x\; 2}}x} + {\frac{{y\; 2x\; 1} - {y\; 1{x2}}}{{x\; 1} - {x\; 2}}( {x \leq {x\; 2}} )}} \\{{\frac{{y\; 2} - {y\; 3}}{{x\; 2} - {x\; 3}}x} + {\frac{{y\; 3x\; 2} - {y\; 2x\; 3}}{{x\; 2} - {x\; 3}}( {x > {x\; 2}} )}}\end{matrix}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$

In the function of Equation 2, a y value when the representative valueof the storage image data is inputted into x becomes the kick-backcorrection gray value.

Hereinafter, a method of generating the auxiliary image data by usingthe kick-back correction gray value will be described.

As shown in Equation 3, a value acquired by subtracting the kick-backcorrection gray value depending on the representative value from therepresentative value of the storage image data is a gray value of theauxiliary image data.Ga=Gr−dG (Ga: gray value of auxiliary image data, Gr: representativevalue, dG: kick-back correction gray value depending on representativevalue)  (Equation 3)

The kick-back corrector 660 transmits the auxiliary image data generatedby using Equation 3 to the data driver 500, and the data driver 500applies an auxiliary voltage corresponding to the auxiliary image datato the data lines D1 to Dm in the vertical blank range at the time ofdisplaying the still image.

The image data which the signal controller 600 transmits to the datadriver 500 is summarized for each case as follows.

The signal controller 600 transmits the input image data transmittedfrom the graphics processing unit 700 to the data driver 500 to drivethe display panel 300 at the first frequency at the time of displayingthe motion picture. The signal controller 600 transmits the storageimage data stored in the frame memory 640 to the data driver 500 todrive the display panel 300 at the second frequency at the time ofdisplaying the still image. Further, the signal controller 600 transmitsthe auxiliary image data correcting the representative value of thestorage image data to the data driver 500 to apply the auxiliary voltageto the data line in the vertical blank range at the time of displayingthe still image.

Next, referring to FIGS. 43 and 44, a principle of reducing leakagecurrent by inputting the auxiliary image data in the vertical blankrange at the time of displaying the still image in the display deviceaccording to the exemplary embodiment will be described.

FIG. 43 is an equivalent circuit diagram for one pixel of the displaydevice according to the exemplary embodiment, and FIG. 44 is a diagramillustrating leakage current when a predetermined voltage is appliedduring a vertical blank range in the display device according to theexemplary embodiment.

As shown in FIG. 43, the switching element Q is formed so that one pixelof the display device according to the exemplary embodiment is connectedto the gate line Gn and the data line Dm. In the switching element Q asa 3-terminal element such as a thin-film transistor, a control terminalis connected with the gate line Gn, an input terminal is connected withthe data line Dm, and an output terminal is connected with a liquidcrystal capacitor Clc.

When the gate-on voltage is applied to the gate line Gn and the datavoltage is applied to the data line Dn, the liquid crystal capacitor Clcis charged. Subsequently, when the gate-off voltage is applied to thegate line Gn to turn off the switching element Q, no current should flowbetween the input terminal and the output terminal of the switchingelement Q. However, leakage current Idp that flows onto the inputterminal from the output terminal of the switching element Q isgenerated due to a characteristic of the switching element Q such as thethin-film transistor. The leakage current Idp is proportionate to adifference between the voltage Vd of the input terminal and the voltageVp of the output terminal of the switching element Q.

In general, because the data voltage is not input in the vertical blankrange between two neighboring frames, a voltage difference between theinput terminal and the output terminal of the switching element Q islarge. The leakage current is increased due to the voltage differencebetween the input terminal and the output terminal of the switchingelement Q when the display panel is driven at a low frequency byincreasing the length of the vertical blank range between two frames.

In the exemplary embodiments, the display panel is driven at the lowfrequency at the time of displaying the still image, and thepredetermined voltage is applied to the data line in the vertical blankrange to reduce the leakage current.

As shown in FIG. 44, the leakage current is changed when data voltagecorresponding to a black gray is applied to the data line and datavoltage corresponding to a white gray is applied to the data line in thevertical blank range.

In this case, the predetermined voltage applied to the data line ispreferably set as a value which is most approximate to the pixel voltagecharged in the liquid crystal capacitor Clc of each pixel, that is, thevoltage of the output terminal of the switching element Q.

According to the exemplary embodiments, the value representing thestorage image data is calculated for each data line and the calculatedvalue is corrected according to the kick-back voltage to generate theauxiliary image data, and thereafter, the auxiliary voltagecorresponding thereto is applied to the data line. Accordingly, thevoltage between the input terminal and the output terminal of theswitching element Q can be minimized, and as a result, the leakagecurrent can also be minimized.

Next, a display device according to the eighth exemplary embodiment willbe described.

The display device according to the eighth exemplary embodiment is thesame as the display device of the first exemplary embodiment such thatthe detailed description is omitted.

A gate voltage applied to a gate line of the display device according tothe eighth exemplary embodiment will be described with reference to FIG.45 and FIG. 46.

FIG. 45 is a diagram illustrating one pixel of the display deviceaccording to the eighth exemplary embodiment, and FIG. 46 is a graphillustrating current between an input terminal and an output terminalaccording to gate voltage in a switching element of the display deviceaccording to the eighth exemplary embodiment.

One pixel of the display device according to the eighth exemplaryembodiment includes a switching element Q connected to a gate line Gnand a data line Dm, and a liquid crystal capacitor Clc and a storagecapacitor Cst connected to the switching element Q. At this time, thecontrol terminal of the switching element Q is connected to the gateline Gn, the input terminal is connected to the data line Dm, and theoutput terminal is connected to the liquid crystal capacitor Clc and thestorage capacitor Cst.

The gate-on voltage and the gate-off voltage are alternately applied tothe gate line Gn to control an on/off state of the switching element Q.

When the gate-on voltage is applied to the gate line Gn, the switchingelement Q enters an on state and current Ids flows between the inputterminal and the output terminal. Accordingly, the pixel electrode ischarged to the pixel voltage Vp by the data voltage Vd supplied throughthe data line Dn.

When the gate-off voltage is applied to the gate line Gn, the switchingelement Q enters an off state and the current Ids does not flow betweenthe input terminal and the output terminal. However, the voltagedifference is formed between the data voltage Vd and the pixel voltageVp, and as a result, the leakage current is generated between the inputterminal and the output terminal. Accordingly, it is preferred that thegate-off voltage is selected as a voltage value that is capable ofminimizing the leakage current.

As shown in FIG. 46, it is verified that a difference in the leakagecurrent when the pixel voltage Vp charged in the pixel electrode ispositive and negative occurs.

FIG. 46 shows the current Ids between the input terminal and the outputterminal according to gate voltage Vg input to the control terminal ofthe switching element Q when the pixel voltage Vp is positive andnegative. FIG. 46 shows the result of the case where the pixel voltageVp is 0 V and the data voltage Vd is 10 V when the pixel voltage Vp isnegative and the pixel voltage Vp is 20 V and the data voltage Vd is 10V when the pixel voltage Vp is positive.

If a voltage that is capable of minimizing the current Ids between theinput terminal and the output terminal of the switching element Q whenthe pixel voltage Vp is negative is selected as the gate-off voltage, adifference in the leakage currents between a positive pixel and anegative pixel occurs. As a result, luminance characteristics of thepositive pixel and the negative pixel are different from each other.Further, If a voltage that is capable of minimizing the current Idsbetween the input terminal and the output terminal of the switchingelement Q when the pixel voltage Vp is positive is selected as thegate-off voltage, a difference in the leakage currents between apositive pixel and a negative pixel occurs. As a result, luminancecharacteristics of the positive pixel and the negative pixel aredifferent from each other.

Accordingly, in the display device according to the exemplaryembodiment, when the leakage currents are the same when the pixelvoltage Vp charged in the pixel electrode is positive and negative, thevoltage of the control terminal of the switching element Q may beselected as the gate-off voltage. For example, the gate-off voltage maybe set to −4 V according to the experimental result shown in FIG. 46. Ofcourse, the values may be variously changed as experimental conditionsare changed.

Hereinafter, a characteristic in which flicker is improved (i.e.,flicker becomes less perceptible) in the display device according to theexemplary embodiment will be described with reference to FIGS. 47 and48.

FIG. 47 is a diagram illustrating a luminance characteristic when astill image is displayed in a display device according to the relatedart, and FIG. 48 is a diagram illustrating a luminance characteristicwhen a still image is displayed in the display device according to theeighth exemplary embodiment. In detail, FIG. 47 is a diagramillustrating a luminance characteristic in the case where a voltage thatis capable of minimizing the current Ids between the input terminal andthe output terminal of the switching element Q when the pixel voltage Vpis negative is selected as the gate-off voltage. FIG. 48 is a diagramillustrating a luminance characteristic in the case where a voltage ofthe control terminal of the switching element Q is selected as thegate-off voltage in the case where the leakage current is the same whenthe pixel voltage Vp is positive and negative.

Because the same image is shown for every frame when the still image isdisplayed, in theory, luminance of each pixel is not changed.

As shown in FIG. 47, in the display device according to the related artin which the voltage that is capable of minimizing the current Idsbetween the input terminal and the output terminal of the switchingelement Q when the pixel voltage Vp is negative is selected as thegate-off voltage, when the still image is displayed, the luminance ofthe entire screen is repeatedly increased and decreased for eachsuccessive frame. As a result, flicker will be noticable.

As described above, the cause for the flicker is because the luminanceof the pixel when the positive pixel voltage is applied differs from theluminance of the pixel when the negative pixel voltage is applied. Theluminance of the pixel to which the positive pixel voltage is appliedand luminance of the pixel to which the negative pixel voltage isapplied are changed for every frame, and thus the luminance of theentire screen is also changed for every frame.

As shown in FIG. 48, when the leakage current is the same when the pixelvoltage Vp is positive and negative, in the display device according tothe exemplary embodiment in which the voltage of the control terminal ofthe switching element Q is selected as the gate-off voltage, theluminance of the entire screen is uniformly maintained when the stillimage is displayed.

In the display device according to the eighth exemplary embodiment,while the still image is displayed, the amplitude of the leakage currentgenerated in the pixel to which the positive pixel voltage is appliedand the amplitude of the leakage current generated in the pixel to whichthe negative pixel voltage is applied are the same, and as a result, thesum value of the luminance of the pixel to which the positive pixelvoltage is applied and the luminance of the pixel to which the negativepixel voltage is applied may be uniformly maintained. Accordingly, theluminance of the entire screen may be uniformly maintained and theflicker may not be perceptible.

The feature in which the flicker can be reduced by setting the gate-offvoltage as the voltage of the control terminal of the switching elementwhen the leakage currents are the same in the positive pixel and thenegative pixel was described above.

Furthermore, by setting the gate-off voltage so as to be in apredetermined range based on the voltage of the control terminal of theswitching element when the leakage currents are the same in the positivepixel and the negative pixel, the exemplary embodiment may have the sameor a similar effect, and hereinafter the range will be described withreference to Table 3 and FIG. 49.

Table 3 is a table illustrating a flicker value according to a gate-offvoltage value when the still image is displayed in the display deviceaccording to the exemplary embodiment, and FIG. 49 is a diagramillustrating Table 3 by a graph. That is, FIG. 49 is a graphillustrating a flicker value according to a gate-off voltage value whena still image is displayed in the display device according to theexemplary embodiment.

As shown in Table 3 and FIG. 49, in the display device according to theexemplary embodiment, the frequency is decreased by changing the lengthof the vertical blank period and a magnitude of a clock frequency, andthe flicker value is changed according to the gate-off voltage when thestill image is displayed at a low frequency.

In the experimental example described in FIG. 46, when the leakagecurrents are the same in the positive pixel and the negative pixel, thevoltage of the control terminal of the switching element Q is −4 V.Accordingly, the flicker is measured by setting a lower voltage and ahigher voltage than −4 V as the gate-off voltage, and the result isshown in Table 3 and FIG. 49.

TABLE 3 Flicker value (dB) Gate-off Vertical blank Clock frequencyvoltage (V) period change change −6.5 −37.9 −36.7 −6 −40 −39.5 −5.5−42.7 −42.4 −5 −46.2 −45 −4.5 −49.4 −46.3 −4 −51.4 −46.4 −3.5 −52.4−45.6 −3 −52.5 −44.7 −2.5 −53.5 −43.7

Referring to Table 3 and FIG. 49, it is verified that a flicker valuewhen the gate-off voltage is −4 V has a similar value to the flickervalue when the gate-off voltage is changed by about −20% or +20% basedon −4 V. Even in the case where the gate-off voltage has a value ofabout +20% or more based on −4 V, the flicker value may also have asimilar value or a lower value than the flicker value when the gate-offvoltage is −4 V. However, in the case where the gate-off voltage may beset to a very high value, the leakage current is increased, and as aresult, a problem such as decolorization and the like may also occur.

Accordingly, it is preferred that the gate-off voltage is set so as tobe in the range of about −20% to +20% based on the voltage of thecontrol terminal of the switching element when the leakage currents arethe same in the positive pixel and the negative pixel. Further, it ismore preferred that the gate-off voltage is set so as to be in the rangeof about −10% to +10% based on the voltage of the control terminal ofthe switching element when the leakage currents are the same in thepositive pixel and the negative pixel.

On the basis of this, the range of the gate-off voltage is representedby equations as follows.

In the display device according to the eighth exemplary embodiment, thegate-off voltage applied to the gate line Gn when the display panel isdriven at the second frequency may be set so as to be in the range ofEquation 4.Va−0.2|Va|≦Voff2≦Va+0.2|Va|  (Equation 4)

(Voff2: the gate-off voltage when the display panel is driven at thesecond frequency, Va: voltage of the control terminal of the switchingelement when leakage current flowing between the input terminal and theoutput terminal of the switching element when the positive pixel voltageis applied to the pixel electrode and leakage current flowing betweenthe input terminal and the output terminal of the switching element whenthe negative pixel voltage is applied to the pixel electrode are thesame)

In the display device according to the eighth exemplary embodiment, thegate-off voltage applied to the gate line Gn when the display panel isdriven at the second frequency may alternatively be set so as to be inthe range of Equation 5.Va−0.1|Va|≦Voff2≦Va+0.1|Va|  (Equation 5)

(Voff2: the gate-off voltage when the display panel is driven at thesecond frequency, Va: voltage of the control terminal of the switchingelement when leakage current flowing between the input terminal and theoutput terminal of the switching element when the positive pixel voltageis applied to the pixel electrode and leakage current flowing betweenthe input terminal and the output terminal of the switching element whenthe negative pixel voltage is applied to the pixel electrode are thesame)

Because the display device according to the eighth exemplary embodimentis driven at a low frequency when the still image is displayed, toprevent flicker from occurring, the gate-off voltage is set according toEquation 4 and Equation 5.

Because the display device according to the eighth exemplary embodimentis driven at a high frequency when the still image is displayed, theflicker is not as evident. As a result, the gate-off voltage may be setto a lower value. That is, the gate-off voltage when the display panelis driven at the first frequency may be set to be lower than thegate-off voltage when the display panel is driven at the secondfrequency.

Unlike this, the gate-off voltage when the display panel is driven atthe first frequency may also be set to be the same as the gate-offvoltage when the display panel is driven at the second frequency. Inthis case, the range of the gate-off voltage is represented by equationsas follows.Va−0.2|Va|≦Voff1≦Va+0.2|Va|  (Equation 6)

(Voff1: the gate-off voltage when the display panel is driven at thefirst frequency, Va: voltage of the control terminal of the switchingelement when leakage current flowing between the input terminal and theoutput terminal of the switching element when the positive pixel voltageis applied to the pixel electrode and leakage current flowing betweenthe input terminal and the output terminal of the switching element whenthe negative pixel voltage is applied to the pixel electrode are thesame)

In the display device according to the exemplary embodiment, thegate-off voltage applied to the gate line Gn when the display panel isdriven at the first frequency may alternatively be set so as to be inthe range of Equation 7.Va−0.1|Va|≦Voff1≦Va+0.1|Va|  (Equation 7)

(Voff1: the gate-off voltage when the display panel is driven at thefirst frequency, Va: voltage of the control terminal of the switchingelement when leakage current flowing between the input terminal and theoutput terminal of the switching element when the positive pixel voltageis applied to the pixel electrode and leakage current flowing betweenthe input terminal and the output terminal of the switching element whenthe negative pixel voltage is applied to the pixel electrode are thesame)

Hereinafter, a method of calculating a flicker value will further bedescribed below with reference to FIGS. 50 and 51.

FIG. 50 is a graph illustrating intensity of light emitted from adisplay panel according to time, and FIG. 51 is a diagram illustratingequipment used for flicker measurement.

The flicker means a phenomenon in which flicker of light is perceived asintensity of light emitted from a screen is not uniform and isperiodically changed over time. When the display device is driven at 60Hz, flickers of 60 times per second occur.

Referring to FIG. 50, the intensity of light is changed according totime. The intensity of light has a value between Vmax and Vmin and isperiodically changed.

A first method of calculating a flicker value is a method of calculatinga ratio of an AC component to a DC component. After measuring the Vmaxand Vmin values, the flicker value may be calculated by using Equation8.

$\begin{matrix}{P = {{\frac{A\; C_{component}}{D\; C_{component}}*100} = {\frac{v_{\max} - v_{\min}}{( {v_{\max}*v_{\min}} )\text{/}2}*100\mspace{14mu}( {F\text{:}\mspace{14mu}{Flicker}\mspace{14mu}{value}} )}}} & ( {{Equation}\mspace{14mu} 8} )\end{matrix}$

Because the sensitivity of eyes is changed according to the intensity oflight, and the change amount is nonlinear, the sensitivity of eyes needsto be considered when the flicker value is calculated. In the case ofthe first method, because the sensitivity of eyes is not considered, acorrect flicker value is not easily derived, but the calculating methodthereof is simple.

Hereinafter, considering a change in the sensitivity of eyes accordingto the intensity of light, a second method used in order to derive amore correct flicker value will be described below.

As shown in FIG. 51, a luminance meter 20 capable of measuring luminanceis disposed at the surface to which light is emitted from a displaydevice 10. The luminance meter 20 may be, for example, BM-7 and thelike. Further, a dynamic signal analyzer (DSA) 30 which receives andprocesses a signal from the luminance meter 20 is connected to theluminance meter 20.

First, the display device 10 is controlled to a state where light can beemitted from the display device 10, and luminance of the light emittedfrom the display device 10 is measured by using the luminance meter 20.The luminance of light measured by the luminance meter 20 has ananalogue value, and the analogue value is transmitted to the dynamicsignal analyzer 30. The dynamic signal analyzer 30 reads a root meansquare value (rms value) of a 0 Hz component and a 30 Hz component fromthe analogue value by a decibel (dB) unit.

After reading the rms value of the 0 Hz component and the 30 Hzcomponent from the dynamic signal analyzer 30, the flicker value may becalculated by using the following Equation 9. Equation 9 was made inconsideration of a size change of pupils in response to the intensity oflight, intensity of light transmitting through the pupils to the sizechange of the pupils, reactivity of eyes to the intensity of lighttransmitting through the pupils, and the like.

$\begin{matrix}{{F = {1000*( {A - B} )}}( {{A = \frac{\lbrack {{\pi( \frac{10^{{{0.8558 - {0.000401{\{{\log{({L{({{0\mspace{14mu}{Hz}} + {30\mspace{14mu}{Hz}}})}})}}\}}} + 0.86})}^{3}}}{2} )}^{2}{L( {{0\mspace{14mu}{Hz}} + {30\mspace{14mu}{Hz}}} )}} \rbrack^{0.74}}{\lbrack {{\pi( \frac{10^{0.8558 - {0.000401{\{{\log{({L{({{0\mspace{14mu}{Hz}} + {30\mspace{14mu}{Hz}}})}})}}\}}} + 0.86^{3}}}{2} )}^{2}{L( {{0\mspace{14mu}{Hz}} + {30\mspace{14mu}{Hz}}} )}} \rbrack^{0.74} + 1584.9^{0.74}}},{B = \frac{\lbrack {{\pi( \frac{10^{{{0.8558 - {0.000401{\{{\log{({L{({{0\mspace{14mu}{Hz}} - {30\mspace{14mu}{Hz}}})}})}}\}}} + 0.86})}^{3}}}{2} )}^{2}{L( {{0\mspace{14mu}{Hz}} + {30\mspace{14mu}{Hz}}} )}} \rbrack^{0.74}}{\lbrack {{\pi( \frac{10^{0.8558 - {0.000401{\{{\log{({L{({{0\mspace{14mu}{Hz}} - {30\mspace{14mu}{Hz}}})}})}}\}}} + 0.86^{3}}}{2} )}^{2}{L( {{0\mspace{14mu}{Hz}} - {30\mspace{14mu}{Hz}}} )}} \rbrack^{0.74} + 1584.9^{0.74}}},{{L( {{0\mspace{14mu}{Hz}} + {30\mspace{14mu}{Hz}}} )} = {\lbrack {10^{\frac{{0\mspace{14mu}{{Hz}{({dB})}}} + b}{20}} + 10^{\frac{{30\mspace{14mu}{{Hz}{({dB})}}} + b}{20}}} \rbrack\text{/}a}},{{L( {{0\mspace{14mu}{Hz}} - {30\mspace{14mu}{Hz}}} )} = {\lbrack {10^{\frac{{0\mspace{14mu}{{Hz}{({dB})}}} + b}{20}} - 10^{\frac{{30\mspace{14mu}{{Hz}{({dB})}}} + b}{20}}} \rbrack\text{/}a}},} } & ( {{Equation}\mspace{14mu} 9} )\end{matrix}$

0 Hz: rms value of 0 Hz component of luminance of light

30 Hz: rms value of 30 Hz component of luminance of light

a: proportional constant to luminance of light inputted to the luminancemeter 20 and outputted voltage

b: reference voltage for calculating voltage inputted to the dynamicsignal analyzer 30 by decibel (dB)

In the case of the second method, the calculating method is morecomplicated, but because the flicker value is calculated by consideringvarious parameters affecting the change in the flicker value, a morecorrect value may be calculated.

The flicker value shown in Table 3 and FIG. 49 is a value calculated byusing the second method.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the disclosure, including the appended claims.

<Description of Symbols>  10: display device  20: luminance meter  30:dynamic signal analyzer 300: display panel 400: gate driver 500: datadriver 600: signal controller 610: signal receiving unit 620: framecounting unit 630: line memory 640: frame memory 650: driving frequencyselecting unit 660: kick-back corrector 700: graphics processing unit800: gray voltage generator 900: light source unit 910: light sourcedriver 912: driving frequency receiving unit 914: light source unitdriving ratio selecting unit 916: light source driving signal generator

What is claimed is:
 1. A display device, comprising: a display panelconfigured to display a still image and a motion picture; a signalcontroller configured to control signals for driving the display panel;a graphics processing unit configured to transmit input image data tothe signal controller; a light source unit configured to irradiate lightto the display panel; and a light source driver configured to controlsignals to drive the light source unit, wherein the signal controllercomprises a frame memory configured to store the input image data, andthe display panel is driven at a first frequency when the motion pictureis displayed and is driven at a second frequency that is lower than thefirst frequency when the still image is displayed, wherein the lightsource driver constantly maintains a driving ratio of the light sourceunit when the display panel is driven with the first frequency, andperiodically changes the driving ratio of the light source unit when thedisplay panel is driven with the second frequency.
 2. The display deviceof claim 1, wherein the graphics processing unit transmits a still imagestart signal and a still image end signal to the signal controller. 3.The display device of claim 2, wherein the signal controller furthercomprises a frame counting unit configured to count the number of stillimage sequential frames inputted before the still image end signal isapplied after the still image start signal is applied, and configured tocount the number of motion picture sequential frames inputted until thestill image start signal is applied after the still image end signal isapplied.
 4. The display device of claim 3, wherein the signal controllerstores the input image data in the frame memory and inactivates atransmission of the input image data from the graphic processing unitwhen the number of the still image sequential frames is equal to or morethan x, and activates the transmission of the input image data from thegraphic processing unit when the number of the motion picture sequentialframes is equal to or more than y.
 5. The display device of claim 4,wherein the signal controller outputs the storage image data stored inthe frame memory to the display panel at the second frequency when thenumber of the still image sequential frames is equal to or more than x,and outputs the input image data to the display panel at the firstfrequency when the number of the motion picture sequential frames isequal to or more than y.
 6. The display device of claim 2, wherein thelight source driver drives the light source unit at a first ratio whenthe display panel is driven at the first frequency, and drives the lightsource unit at a second ratio when the display panel is driven at thesecond frequency.
 7. The display device of claim 6, wherein the secondratio is lower than the first ratio when the display panel is a normallyblack mode, and the second ratio is higher than the first ratio when thedisplay panel is a normally white mode.
 8. The display device of claim7, wherein the signal controller further comprises a signal receivingunit configured to transmit the input image data from the graphicsprocessing unit, and a driving frequency selecting unit configured toselect the first frequency when the still image is displayed andconfigured to select the second frequency when the motion picture isdisplayed.
 9. The display device of claim 7, wherein the light sourcedriver comprises a driving frequency receiving unit configured toreceive a driving frequency of the display panel from the signalcontroller, a light source unit driving ratio selecting unit configuredto determine the driving ratio of the light source unit according to thedriving frequency, and a light source driving signal generatorconfigured to generate a signal for driving the light source accordingto the driving ratio of the light source unit.
 10. The display device ofclaim 2, wherein the display panel is a normally black mode, and thelight source driver drives the light source unit with the first ratiowhen the display panel is driven with the first frequency, and drivesthe light source unit with the first ratio and a ratio that issequentially decreased from the first ratio when the display panel isdriven with the second frequency.
 11. The display device of claim 10,wherein the display panel comprises: a gate line and a data line; a gatedriver configured to drive a gate line; and a data driver configured todrive a data line, and the signal controller transmits an STV signal tothe gate driver at a start position of every frame.
 12. The displaydevice of claim 11, wherein the light source driver drives the lightsource unit with the first ratio as a position where the STV signal istransmitted when the display panel is driven with the second frequency,and drives the light source unit with a ratio that is sequentiallydecreased from the first ratio before a next STV signal is transmitted.13. The display device of claim 12, wherein a transmission cycle of theSTV signal when the display panel is driven with the first frequency isthe same as a change cycle of the driving ratio of the light source unitwhen the display panel is driven with the second frequency.
 14. Thedisplay device of claim 2, wherein the display panel is a normally whitemode, and the light source driver drives the light source unit with thefirst ratio when the display panel is driven with the first frequencyand drives the light source unit with the first ratio and a ratio thatis sequentially increased from the first ratio when the display panel isdriven with the second frequency.
 15. A method for driving a displaydevice comprising a display panel displaying a moving picture and astill image, and a signal controller controlling signals to drive thedisplay panel, comprising: transmitting input image data and driving adisplay panel with a first frequency; applying a still image startsignal; changing a driving frequency of the display panel into a secondfrequency that is lower than the first frequency; applying a still imageend signal; and changing a driving frequency of the display panel intothe first frequency, wherein a light source driver constantly maintainsa driving ratio of a light source unit when the display panel is drivenwith the first frequency and periodically changes the driving ratio ofthe light source unit when the display panel is driven with the secondfrequency.
 16. The method of claim 15, further comprising: counting thenumber of still image sequential frames inputted before the still imageend signal is applied after the still image start signal is applied; andcounting the number of motion picture sequential frames inputted untilthe still image start signal is applied after the still image end signalis applied.
 17. The method of claim 16, wherein the signal controllerstores the input image data in the frame memory and inactivates atransmission of the input image data from the graphic processing unitwhen the number of the still image sequential frames is equal to or morethan x, and activates the transmission of the input image data from thegraphic processing unit when the number of the motion picture sequentialframes is equal to or more than y.
 18. The method of claim 17, whereinthe signal controller outputs the storage image data stored in the framememory to the display panel at the second frequency when the number ofthe still image sequential frames is equal to or more than x, andoutputs the input image data to the display panel at the first frequencywhen the number of the motion picture sequential frames is equal to ormore than y.
 19. The method of claim 15, wherein when the display panelis driven at a first frequency, the light source unit is driven at thefirst ratio, and when the display panel is driven at a second frequency,the light source unit is driven at the second ratio.
 20. The method ofclaim 19, wherein when the display panel is a normally black mode, thesecond ratio has a lower value than the first ratio, and when thedisplay panel is a normally white mode, the second ratio has a highervalue than the first ratio.
 21. The method of claim 19, wherein thedriving ratio of the light source unit according to the drivingfrequency of the display panel is selected by using a look-up table or afunction.
 22. The method of claim 19, wherein the conversion of thedriving frequency of the display panel and the driving ratio of thelight source unit is performed in a vertical blank period.
 23. Themethod of claim 15, wherein the display panel is a normally black mode,when the display panel is driven with the first frequency, the lightsource unit is driven with the first ratio, and when the display panelis driven with the second frequency, the light source unit is drivenwith the first ratio and a ratio that is sequentially decreased from thefirst ratio.
 24. The method of claim 23, wherein the display panelcomprises a gate line and a data line, the display device furthercomprises a gate driver configured to drive the gate line and a datadriver configured to drive the data line, and the signal controllertransmits an STV signal to the gate driver at the start position everyframe.
 25. The method of claim 24, wherein when the display panel isdriven with the second frequency, the light source unit is driven withthe first ratio at a position transmitting the STV signal, and the lightsource unit is driven with a ratio that is sequentially decreased fromthe first ratio before transmission of a next STV signal.
 26. The methodof claim 25, wherein a transmission cycle of the STV signal when thedisplay panel is driven with the first frequency is the same as a changecycle of the driving ratio of the light source unit when the displaypanel is driven with the second frequency.
 27. The method of claim 15,wherein the display panel is a normally white mode, when the displaypanel is driven with the first frequency, the light source unit isdriven with the first ratio, and when the display panel is driven withthe second frequency, the light source unit is driven with the firstratio and a ratio that is sequentially increased from the first ratio.